Pneumonia is an inflammatory infection of the lung parenchyma, primarily affecting the alveoli—tiny air sacs responsible for oxygen and carbon dioxide exchange [1]. During infection, these alveoli fill with fluid, pus, or inflammatory material, impairing breathing and oxygen absorption [2]. The condition can affect one or both lungs and ranges from mild to life-threatening, often requiring hospitalization in severe cases [1]. Pneumonia can be caused by various pathogens, including bacteria such as Streptococcus pneumoniae, viruses like the influenza virus and SARS-CoV-2, and fungi such as Pneumocystis jirovecii [4]. It poses a greater risk to vulnerable populations, including children under five, adults over 65, and individuals with chronic conditions like chronic obstructive pulmonary disease or weakened immune systems [5]. Diagnosis typically involves a combination of clinical evaluation, chest X-ray, and laboratory tests such as blood analysis and sputum culture [6]. Treatment depends on the causative agent: bacterial pneumonia is managed with antibiotics like amoxicillin or levofloxacin, while viral pneumonia may require antiviral drugs or supportive care [7]. Preventive strategies include vaccination with pneumococcal conjugate vaccine (PCV13, PCV20) and influenza vaccine, along with hygiene practices and smoking cessation [8]. Global health efforts, led by organizations like the World Health Organization, emphasize reducing pneumonia-related mortality through improved access to vaccines, early diagnosis, and appropriate treatment, especially in low-resource settings [5].

Etiology and Pathogens

Pneumonia is an inflammatory infection of the lung parenchyma primarily caused by a diverse range of pathogens, including bacteria, viruses, fungi, and, in some cases, the aspiration of foreign substances such as gastric contents [4]. The specific causative agent varies depending on the environment in which the infection is acquired (community versus healthcare settings), the patient's age, and their underlying health or immunological status. Understanding the etiology is crucial for guiding appropriate diagnostic and therapeutic strategies.

Bacterial Causes

Bacterial pneumonia is a leading cause of the disease, particularly in adults. The most common bacterial pathogen is Streptococcus pneumoniae (pneumococcus), which is the primary cause of community-acquired pneumonia (CAP) [11]. This bacterium is a major target for preventive strategies, including the pneococcal conjugate vaccine. Other frequent bacterial agents include Haemophilus influenzae, especially in individuals with chronic lung diseases like chronic obstructive pulmonary disease, and atypical bacteria such as Mycoplasma pneumoniae, which causes a milder form known as "atypical pneumonia" [11]. Legionella pneumophila, the bacterium responsible for Legionnaires' disease, is another significant cause of CAP [11].

In healthcare settings, particularly in hospitals or intensive care units (ICUs), the bacterial landscape shifts toward more virulent and often multidrug-resistant organisms. Common pathogens in these environments include Staphylococcus aureus, including methicillin-resistant strains (MRSA), Pseudomonas aeruginosa, and Acinetobacter spp. [11]. P. aeruginosa is especially relevant in patients with underlying conditions such as cystic fibrosis, bronchiectasis, or those who have received prior antibiotic therapy, and its resistance to multiple antibiotics, including carbapenems, poses a significant clinical challenge [15].

Viral Causes

Viruses are a frequent cause of pneumonia, particularly in children and during seasonal outbreaks of respiratory infections. The most common viral pathogens include the respiratory syncytial virus (RSV), which is a leading cause of pneumonia in infants and young children [5]. Influenza viruses (the cause of the flu) can lead to severe pneumonia, especially in vulnerable populations [2]. Other respiratory viruses, such as adenovirus, parainfluenza virus, and, more recently, the SARS-CoV-2 virus, which causes COVID-19, are also significant etiological agents [18]. Viruses can cause primary viral pneumonia or predispose individuals to secondary bacterial infections, complicating the clinical course. The identification of the specific virus is critical for implementing appropriate antiviral therapy and infection control measures.

Fungal Causes

Fungal pneumonia is less common and typically affects individuals with weakened immune systems, such as patients with HIV/AIDS, organ transplant recipients, or those undergoing chemotherapy. Key fungal pathogens include Pneumocystis jirovecii, which causes a life-threatening infection in immunocompromised hosts, Histoplasma capsulatum, and species of Candida [4]. These infections often require specific antifungal treatments and are managed differently from bacterial or viral pneumonias. The diagnosis of fungal pneumonia can be challenging and may require specialized testing, such as sputum analysis or bronchoalveolar lavage, to identify the causative organism.

Pathogen Distribution in Different Settings

The distribution of pathogens differs significantly between community-acquired and healthcare-associated pneumonia. CAP is predominantly caused by S. pneumoniae, H. influenzae, and atypical bacteria like Mycoplasma pneumoniae and Chlamydia pneumoniae [20]. In contrast, healthcare-associated pneumonia, including hospital-acquired and ventilator-associated pneumonia (VAP), is characterized by a higher prevalence of multidrug-resistant (MDR) pathogens such as P. aeruginosa, MRSA, and extended-spectrum beta-lactamase (ESBL)-producing Enterobacterales [21]. This distinction is vital for selecting empirical antibiotic therapy, as treatment for healthcare-associated infections must cover these resistant organisms from the outset [22]. Despite advances in diagnostics, the causative agent remains unidentified in a substantial proportion of cases, complicating targeted treatment [4].

Clinical Presentation and Differential Diagnosis

The clinical presentation of pneumonia varies widely depending on the causative agent, the patient's age, and underlying health status, but it typically involves a constellation of respiratory and systemic symptoms resulting from inflammation and consolidation of the lung parenchyma [1]. Accurate diagnosis requires distinguishing pneumonia from other common respiratory conditions with overlapping symptoms, such as bronchitis, influenza, and the common cold, based on the severity, localization, and specific clinical features.

Key Clinical Symptoms and Signs

The hallmark symptoms of pneumonia include fever, cough (often productive with purulent or blood-tinged sputum), dyspnea, and pleuritic chest pain that worsens with breathing or coughing [1]. Additional systemic signs such as chills, excessive sweating, fatigue, and loss of appetite are common. Headache, nausea, vomiting, and diarrhea may also occur, particularly in bacterial pneumonia [1]. Confusion or altered mental status is a critical presenting symptom, especially in older adults, and can sometimes be the primary indicator of the disease in this population [27].

In young children, the clinical picture may be less specific and include irritability, fever, cough, difficulty feeding, and tachypnea [5]. The physical examination often reveals abnormal lung sounds, such as crackles or bubbling noises, detected with a stethoscope, which suggest pulmonary inflammation and fluid accumulation in the alveoli [29]. The presence of persistent high fever, severe dyspnea, and productive cough with thick sputum should prompt immediate medical evaluation [30].

Differentiating Pneumonia from Other Respiratory Infections

Pneumonia is distinguished from other respiratory infections by the site of infection and the severity of illness. While conditions like bronchitis, influenza, and the common cold primarily affect the upper airways, pneumonia is an infection of the alveoli and pulmonary parenchyma, leading to impaired gas exchange and more severe clinical manifestations [1].

  • Bronchitis: This condition involves inflammation of the bronchial tubes, usually caused by viruses, and is characterized by a productive cough. However, it does not involve the alveoli and is generally less severe than pneumonia. It typically resolves without complications and does not require antibiotic treatment, unlike bacterial pneumonia, which may necessitate hospitalization and antimicrobial therapy [32] [33].

  • Influenza (Flu): Influenza is a viral infection of the upper and lower respiratory tracts with a sudden onset of high fever, muscle aches, dry cough, and profound fatigue. While it can progress to pneumonia, influenza itself does not cause the alveolar filling with fluid or pus that defines pneumonia. Pneumonia is a more serious complication with a higher risk of mortality, particularly in vulnerable populations [34].

  • Common Cold: This is a mild viral infection characterized by nasal congestion, sneezing, and a sore throat. It typically lasts 3 to 7 days and does not require specific treatment or antibiotics. In contrast, pneumonia can be life-threatening and may require intensive medical care, including hospitalization for severe cases [35].

The key differentiating factor is the presence of lower respiratory tract involvement and signs of lung consolidation in pneumonia, which are absent in bronchitis, influenza, and the common cold. The risk of complications is significantly higher with pneumonia, especially in children under five, adults over 65, and individuals with compromised immune systems [36].

Atypical Presentations and Vulnerable Populations

The clinical presentation can be atypical in certain high-risk groups. In older adults, the classic signs of fever and cough may be absent, with confusion or a general decline in function being the primary symptoms [27]. This can lead to delayed diagnosis and treatment. Similarly, in young children, symptoms can be non-specific, making early recognition challenging [5]. The presence of comorbidities such as chronic obstructive pulmonary disease, diabetes, or heart disease can also alter the presentation and increase the severity of the illness [39]. Recognizing these atypical patterns is crucial for timely intervention and improved outcomes.

Diagnosis and Diagnostic Imaging

The diagnosis of pneumonia relies on a comprehensive approach that integrates clinical evaluation, laboratory testing, and diagnostic imaging to confirm the presence of pulmonary infection and assess its severity. Among these, diagnostic imaging, particularly chest radiography, plays a pivotal role in establishing a definitive diagnosis and differentiating pneumonia from other respiratory conditions [40]. The identification of new or progressive pulmonary infiltrates on imaging is a cornerstone of diagnostic confirmation, complementing clinical symptoms such as fever, productive cough, and dyspnea [41].

Clinical Evaluation and Initial Diagnostic Steps

The diagnostic process begins with a thorough clinical assessment, including a detailed patient history and physical examination. The history focuses on the onset and nature of symptoms, potential exposure to pathogens, and the presence of risk factors such as age over 65, chronic conditions like chronic obstructive pulmonary disease, or immunosuppression [6]. During the physical exam, a healthcare provider uses a stethoscope to listen for abnormal lung sounds, such as crepitant rales or bronchial breath sounds, which are indicative of alveolar consolidation [29]. To evaluate the severity of the disease and the patient's oxygenation status, non-invasive monitoring such as pulse oximetry is routinely performed to measure blood oxygen saturation [6].

Radiographic Diagnosis: Chest X-ray and CT Scan

The chest X-ray is the primary and most frequently used imaging modality for diagnosing pneumonia. It is instrumental in visualizing the presence of pulmonary infiltrates or consolidations, which appear as areas of increased opacity in the lung parenchyma, confirming the alveolar filling characteristic of the disease [6]. A key radiological sign of alveolar consolidation is the "air bronchogram," which occurs when air-filled bronchi are visualized within a consolidated lung area, providing strong evidence for a pneumonia diagnosis [46]. The distribution of the infiltrate—whether lobar, segmental, or multilobar—can also offer clues about the likely causative pathogen and the type of pneumonia (e.g., community-acquired vs. hospital-acquired) [47].

In cases where the chest X-ray is inconclusive, the clinical presentation is atypical, or the patient is not responding to initial treatment, a computed tomography (CT) scan of the chest is the next step. CT imaging offers superior sensitivity and spatial resolution, allowing for a more detailed assessment of the extent and pattern of lung involvement [48]. It is particularly valuable for identifying complications such as lung abscesses, empyema, or organizing pneumonia, which may present with specific patterns like the "reversed halo sign" [49]. For patients with severe viral pneumonia, such as that caused by SARS-CoV-2, CT scans are crucial for identifying characteristic findings like ground-glass opacities and for monitoring disease progression or the development of long-term sequelae such as pulmonary fibrosis [50].

Advanced Imaging and Functional Assessment

Beyond structural imaging, the assessment of pulmonary function is critical for evaluating the impact of pneumonia on respiratory health, especially in patients with pre-existing lung diseases. pulmonary function tests (PFTs) are not used for initial diagnosis but are essential for monitoring recovery and detecting residual functional impairment. These tests measure lung volumes, airflow rates, and gas exchange capacity, such as the diffusing capacity of the lung for carbon monoxide (DLCO) [51]. Studies have shown that a significant proportion of patients, particularly those who had severe pneumonia from COVID-19, exhibit persistent reductions in DLCO even after clinical recovery, indicating ongoing alveolar damage [52]. In children, PFTs can reveal subtle functional deficits following a bout of community-acquired pneumonia that might not be apparent from clinical symptoms alone, guiding the need for pulmonary rehabilitation [53].

Integration of Imaging with Clinical and Microbiological Data

The interpretation of diagnostic imaging must always be integrated with clinical and laboratory findings to avoid misdiagnosis. For example, the presence of an infiltrate on a chest X-ray is not exclusive to infectious pneumonia and can be seen in non-infectious conditions such as pneumonitis, which is inflammation caused by chemical, allergic, or drug-induced injury [54]. Therefore, a detailed patient history is essential to rule out these alternative etiologies. Similarly, the absence of an infiltrate on an initial X-ray does not completely rule out pneumonia, particularly in the early stages of the disease or in dehydrated patients. In such cases, a follow-up X-ray or a CT scan may be necessary for a definitive diagnosis [41]. The combination of imaging with microbiological data from sputum culture, blood culture, or molecular tests like PCR allows for a precise etiological diagnosis, which is vital for guiding targeted antimicrobial therapy and improving patient outcomes [56].

Treatment and Antimicrobial Therapy

The treatment of pneumonia is tailored to the causative agent, severity of illness, and the patient's overall health status. Antimicrobial therapy is a cornerstone of management, particularly for bacterial infections, while supportive care and antiviral drugs are essential for viral forms. The approach must balance effective pathogen eradication with the need to minimize antimicrobial resistance and adverse effects.

Empiric Antibiotic Therapy for Bacterial Pneumonia

The choice of initial, or empiric, antibiotic therapy depends on the clinical context—whether the pneumonia is community-acquired (CAP), healthcare-associated (HCAP), or hospital-acquired, including ventilator-associated pneumonia (VAP). In CAP, the most common bacterial pathogen is Streptococcus pneumoniae, which remains the primary target of empiric regimens [11]. For outpatients without comorbidities or risk factors for resistant organisms, first-line treatment includes oral amoxicillin or a macrolide such as azithromycin in regions with low resistance rates [58]. In hospitalized patients, intravenous beta-lactams like ceftriaxone or cefotaxime are often combined with a macrolide or a respiratory fluoroquinolone such as levofloxacin to cover both typical and atypical bacteria [59].

In HCAP and hospital-acquired pneumonia, the risk of infection with multidrug-resistant (MDR) organisms increases significantly. Pathogens such as Pseudomonas aeruginosa, Acinetobacter spp.], and methicillin-resistant Staphylococcus aureus (MRSA) necessitate broader coverage. Initial therapy often includes antipseudomonal agents like piperacillin-tazobactam, ceftazidime, or carbapenems such as meropenem, frequently combined with an aminoglycoside (e.g., amikacin) or an antipseudomonal fluoroquinolone [60]. If MRSA is suspected, vancomycin or linezolid should be added to the regimen [60].

Treatment of Atypical and Viral Pneumonia

Pneumonia caused by atypical bacteria such as Mycoplasma pneumoniae, Chlamydia pneumoniae, and Legionella pneumophila requires different antimicrobial agents. These organisms are not effectively targeted by beta-lactams due to their intracellular nature and lack of a cell wall. Instead, macrolides, tetracyclines, or fluoroquinolones are used, as they achieve high intracellular concentrations [62]. For example, azithromycin is a preferred agent for M. pneumoniae due to its favorable pharmacokinetics and anti-inflammatory properties [62].

Viral pneumonia, caused by pathogens such as the influenza virus, respiratory syncytial virus (RSV), or SARS-CoV-2, does not respond to antibiotics. Treatment relies on supportive care and specific antiviral drugs when available. Early administration of antivirals like oseltamivir for influenza or remdesivir for severe COVID-19 can reduce disease severity and complications [41]. Supportive measures include adequate rest, hydration, and the use of antipyretics and humidifiers to alleviate symptoms [65].

Role of Antimicrobial Resistance and Stewardship

Antimicrobial resistance is a major challenge in the treatment of pneumonia, particularly in hospital settings. Resistance in S. pneumoniae to penicillin, macrolides, and trimethoprim-sulfamethoxazole varies geographically and necessitates local surveillance to guide empiric therapy [66]. In Colombia, for instance, increasing resistance to erythromycin has been observed [67]. Similarly, P. aeruginosa is classified as a high-priority pathogen by the World Health Organization due to its resistance to multiple drug classes, including carbapenems [68].

To combat resistance, antimicrobial stewardship programs (ASPs) are critical. These programs promote the rational use of antibiotics through strategies such as local guideline adaptation, rapid microbiological diagnosis, and de-escalation of therapy once the pathogen is identified [69]. The use of biomarkers like procalcitonin can help distinguish bacterial from viral infections and guide the duration of antibiotic therapy, reducing unnecessary exposure [70]. Monotherapy with a beta-lactam is often non-inferior to combination therapy in CAP without risk factors for atypical or resistant pathogens, further minimizing selective pressure for resistance [71].

Duration and De-escalation of Therapy

The duration of antibiotic treatment should be as short as possible while ensuring clinical cure. For most cases of CAP, a 5- to 7-day course is sufficient, guided by clinical response and biomarker trends [72]. In severe cases or those with complications, longer courses may be necessary. Once microbiological results are available, therapy should be de-escalated from broad-spectrum to narrow-spectrum agents to target the specific pathogen and reduce collateral damage to the microbiome [73].

Supportive Care and Adjunctive Therapies

Supportive care is vital for all patients with pneumonia. This includes ensuring adequate hydration, oxygen supplementation for those with hypoxemia, and nutritional support. In severe cases, mechanical ventilation may be required, and strategies such as protective lung ventilation with low tidal volumes (6 mL/kg of ideal body weight) and appropriate positive end-expiratory pressure (PEEP) are used to prevent ventilator-induced lung injury [74]. For patients with refractory hypoxemia, advanced therapies like extracorporeal membrane oxygenation (ECMO) may be considered [75].

Adjunctive therapies such as systemic corticosteroids have shown benefit in severe CAP, particularly in reducing the duration of hospitalization and the need for mechanical ventilation [76]. However, their use must be carefully balanced against the risk of immunosuppression, especially in patients with underlying conditions.

Prevention and Vaccination Strategies

Preventing pneumonia requires a comprehensive approach that combines vaccination, hygiene practices, lifestyle modifications, and infection control, particularly in high-risk populations and healthcare settings. These strategies are essential to reduce the global burden of pneumonia, which remains a leading cause of morbidity and mortality, especially among children under five and adults over 65 [5].

Vaccination: The Cornerstone of Prevention

Vaccination is the most effective strategy for preventing pneumonia, particularly against major pathogens such as Streptococcus pneumoniae and influenza virus. The availability of conjugate and polysaccharide vaccines has significantly reduced the incidence of invasive pneumococcal disease and pneumonia in both children and adults [8].

Pneumococcal Vaccines

Pneumococcal vaccines are designed to protect against infections caused by Streptococcus pneumoniae, the leading bacterial cause of community-acquired pneumonia. Several types of vaccines are available:

  • Pneumococcal conjugate vaccines (PCVs): These include PCV13, PCV15, and PCV20, which protect against 13, 15, and 20 serotypes of pneumococcus, respectively [79]. PCV20, in particular, is recommended for adults aged 50 and older and for individuals with certain chronic conditions or immunocompromising states [80]. In children, PCV13 or PCV20 is included in national immunization schedules and administered during the first year of life [81].

  • Pneumococcal polysaccharide vaccine (PPSV23): This vaccine covers 23 serotypes and is primarily recommended for adults over 65 years of age and those with chronic diseases such as chronic obstructive pulmonary disease, diabetes, or immunodeficiencies [82]. Sequential administration of conjugate and polysaccharide vaccines may be advised in some high-risk individuals to broaden protection.

Recent advances include the development of a 21-valent conjugate vaccine (V116), approved by the U.S. FDA and the European Commission, which offers expanded coverage and has shown robust immunogenicity in clinical trials [83]. Additionally, research is ongoing into protein-based pneumococcal vaccines targeting conserved antigens like PcpA and PhtD, which could provide serotype-independent protection [84].

Influenza Vaccine

Annual vaccination against the influenza virus is critical, as influenza can lead to primary viral pneumonia or predispose individuals to secondary bacterial pneumonia. The vaccine is recommended for all individuals aged six months and older, with particular emphasis on high-risk groups such as older adults, pregnant women, and those with chronic medical conditions [39]. Studies show that influenza vaccination reduces the risk of hospitalization due to pneumonia and severe respiratory illness, especially in the elderly [86].

Respiratory Syncytial Virus (RSV) Vaccine

The virus sincitial respiratorio (VSR) is a major cause of pneumonia in infants and young children. In recent years, vaccines and monoclonal antibodies targeting the prefusion form of the RSV F glycoprotein have been developed. The monoclonal antibody nirsevimab (Beyfortus) has been approved for use in infants and provides passive immunity, reducing hospitalizations by up to 90% [87]. Maternal RSV vaccination is also being implemented to protect newborns during their most vulnerable period. mRNA-based RSV vaccines, such as Moderna’s mRESVIA®, have received regulatory approval in several countries, marking a significant advancement in respiratory vaccine technology [88].

Non-Pharmacological Preventive Measures

In addition to vaccination, several non-pharmacological strategies are vital for pneumonia prevention.

Hygiene and Infection Control

Frequent handwashing with soap and water or alcohol-based hand sanitizers is a simple yet effective way to prevent the spread of respiratory pathogens [89]. Respiratory hygiene, such as covering the mouth and nose when coughing or sneezing with the elbow or a tissue, also helps reduce transmission. In healthcare settings, strict adherence to hand hygiene protocols and the use of personal protective equipment are essential to prevent nosocomial infections, including ventilator-associated pneumonia (VAP) [90].

Smoking Cessation

Tobacco use is a major risk factor for pneumonia, as it damages the respiratory epithelium and impairs mucociliary clearance, increasing susceptibility to infection [91]. Smoking cessation significantly reduces the risk of respiratory infections and improves outcomes in patients with chronic lung diseases like COPD.

Healthy Lifestyle and Nutrition

Maintaining a balanced diet, engaging in regular physical activity, and ensuring adequate sleep strengthen the immune system and enhance resistance to infections [92]. In children, exclusive breastfeeding for the first six months of life provides passive immunity and reduces the incidence of respiratory infections [93].

Prevention in Healthcare Settings

In hospitals and long-term care facilities, specific measures are required to prevent healthcare-associated pneumonia, particularly in critically ill patients on mechanical ventilation.

Ventilator Care Bundles

The implementation of ventilator care bundles has been shown to reduce the incidence of VAP by up to 50%. Key components include:

  • Elevating the head of the bed to 30–45 degrees to prevent aspiration [94].
  • Performing oral care with chlorhexidine to reduce oropharyngeal bacterial colonization [95].
  • Using endotracheal tubes with subglottic suction ports to remove secretions above the cuff [96].
  • Implementing daily sedation interruption and spontaneous breathing trials to facilitate early weaning from mechanical ventilation [97].

Infection Control Protocols

Rigorous infection control practices, including hand hygiene, proper disinfection of ventilator circuits, and isolation precautions for patients colonized with multidrug-resistant organisms, are essential to prevent cross-transmission in intensive care units [98].

Impact of Vaccination Programs on Public Health

The introduction of pneumococcal conjugate vaccines has led to a dramatic reduction in pneumococcal disease. In Spain, for example, these vaccines have halved the incidence of severe pneumococcal infections [99]. Similarly, influenza vaccination has been associated with decreased hospitalization rates and mortality, particularly in high-risk populations [86].

Combining pneumococcal and influenza vaccination enhances protection, especially in older adults. Public health campaigns, such as Mexico’s 2025–2026 winter vaccination program, which includes over 50 million doses of vaccines against pneumococcus, influenza, and SARS-CoV-2, exemplify integrated strategies to boost population immunity [101].

Conclusion

Effective prevention of pneumonia relies on a multifaceted strategy that includes timely vaccination, hygiene promotion, lifestyle modifications, and rigorous infection control in healthcare environments. The continuous development of new vaccines, such as broader-valency pneumococcal vaccines and mRNA-based RSV vaccines, promises to further reduce the global burden of pneumonia. Ensuring equitable access to these interventions, particularly in low-resource settings, remains a critical challenge for public health systems worldwide [5].

Risk Factors and Vulnerable Populations

Pneumonia disproportionately affects certain groups due to a combination of age-related physiological changes, underlying health conditions, and socioeconomic factors. Understanding these risk factors is crucial for targeted prevention and early intervention strategies. The primary populations at increased risk include young children, older adults, individuals with chronic diseases, and those with compromised immune systems [5].

Age-Related Vulnerability

Age is one of the most significant determinants of pneumonia risk. The very young and the elderly are particularly susceptible due to inherent weaknesses in their immune defenses.

Children under five years old, especially infants under one year, are among the most vulnerable groups. Globally, pneumonia is a leading cause of death in this age group, causing an estimated 740,180 deaths in children under five in 2019, accounting for 14% of all child deaths [5]. Their immature immune systems and smaller airways make them more prone to respiratory infections. Key risk factors in this population include lack of exclusive breastfeeding, malnutrition, and exposure to environmental pollutants like secondhand smoke [93].

Adults over 65 years old face a dramatically increased risk of developing pneumonia, with the risk rising further with age. In Spain, it is estimated that 92% of pneumonia-related deaths occur in individuals over 70 years of age, highlighting the severity of the disease in the elderly [106]. This vulnerability is attributed to immunosenescence, the age-related decline in immune function, which impairs the body's ability to fight off infections [107]. Other age-specific factors include a higher prevalence of comorbidities and an increased risk of aspiration due to swallowing difficulties (dysphagia) [108].

Chronic Medical Conditions

The presence of chronic diseases significantly increases the risk of contracting pneumonia and experiencing more severe outcomes. These conditions often impair lung function, weaken the immune system, or disrupt normal protective mechanisms.

  • Chronic obstructive pulmonary disease (COPD): Patients with COPD have damaged airways and impaired mucociliary clearance, making it easier for pathogens to colonize the lungs. Haemophilus influenzae is a frequent cause of infection in this group [109]. The risk of pneumonia is further elevated in those with a history of frequent exacerbations or hospitalizations [110].
  • Cardiovascular diseases: Conditions like heart failure can lead to pulmonary congestion, creating an environment conducive to infection.
  • Diabetes mellitus: Poorly controlled diabetes can impair neutrophil function, a key component of the immune response to bacterial invaders [111].
  • Renal and hepatic diseases: These conditions can lead to a general state of immunosuppression and fluid imbalance, increasing susceptibility.
  • Asthma: While primarily an inflammatory disease, poorly controlled asthma can increase the risk of respiratory infections, including pneumonia [112].

Immunosuppression

Individuals with weakened immune systems are at a heightened risk for pneumonia, including infections caused by opportunistic pathogens that rarely affect healthy people.

  • HIV/AIDS: This condition severely compromises the immune system, making patients highly susceptible to a wide range of infections, including those caused by Pneumocystis jirovecii, a fungus that is a common cause of pneumonia in immunocompromised individuals [4].
  • Cancer and chemotherapy: Cancer itself and its treatments, such as chemotherapy and radiation, can suppress bone marrow function and reduce white blood cell counts, weakening the body's defenses [114].
  • Organ transplant recipients: Patients taking immunosuppressive drugs to prevent organ rejection are at a high risk for various infections, including pneumonia [114].

Lifestyle and Environmental Factors

Several modifiable lifestyle and environmental factors can increase the risk of pneumonia.

  • Smoking: Tobacco use is a major risk factor, as it damages the cilia in the airways, which are responsible for clearing mucus and pathogens. This damage allows bacteria to more easily invade the lung tissue [1].
  • Excessive alcohol consumption: Chronic alcohol abuse can impair the gag reflex, increasing the risk of aspiration pneumonia, and can also weaken the immune system [39].
  • Poor oral hygiene: This can lead to an increased bacterial load in the mouth, which can be aspirated into the lungs [39].
  • Environmental exposures: Living in crowded conditions (hacinamiento) or in homes with high levels of indoor air pollution (e.g., from cooking with solid fuels) facilitates the spread of respiratory pathogens [93].

Socioeconomic Determinants

Socioeconomic status plays a critical role in pneumonia risk and outcomes. Individuals in low-income settings face a higher burden of disease due to a confluence of factors.

  • Limited access to healthcare: This can delay diagnosis and treatment, leading to more severe disease and higher mortality [120].
  • Inadequate vaccination: Poverty and lack of access to health services often result in incomplete vaccination schedules, leaving individuals unprotected against preventable causes of pneumonia like Streptococcus pneumoniae and Haemophilus influenzae type b (Hib) [121].
  • Poor nutrition and living conditions: Malnutrition weakens the immune system, while overcrowded and poorly ventilated homes promote the transmission of infectious agents [5].

These interconnected risk factors underscore the importance of a multifaceted approach to prevention, including widespread vaccination programs, smoking cessation initiatives, improved access to healthcare, and public health measures to address the underlying social determinants of health.

Complications and Prognosis

Pneumonia can lead to a range of complications, particularly in individuals with underlying health conditions, advanced age, or weakened immune systems. The severity of these complications often correlates with the patient's age, comorbidities, and the causative pathogen. Early recognition and management are crucial to improving outcomes and reducing mortality [1].

Common Complications of Pneumonia

One of the most frequent and serious complications is pleural effusion, where fluid accumulates in the pleural space surrounding the lungs. If this fluid becomes infected, it can progress to empyema, a condition requiring drainage and prolonged antibiotic therapy [124]. Another life-threatening complication is septic shock, which occurs when the infection triggers a systemic inflammatory response leading to dangerously low blood pressure and organ failure. This is particularly common in severe cases of bacterial pneumonia, such as those caused by Streptococcus pneumoniae or Staphylococcus aureus [125].

In some cases, pneumonia can result in the formation of a lung abscess, a pus-filled cavity within the lung tissue, often associated with aspiration or infection by anaerobic bacteria. Additionally, respiratory failure may develop, necessitating mechanical ventilation, especially in patients with pre-existing lung diseases like chronic obstructive pulmonary disease [124]. The acute respiratory distress syndrome (ARDS) is another potential complication, characterized by widespread inflammation in the lungs, leading to severe hypoxemia and diffuse alveolar damage, commonly seen in viral pneumonias such as those caused by SARS-CoV-2 [127].

Prognostic Factors and Mortality Risk

The prognosis of pneumonia depends on several factors, including the patient's age, underlying health status, and the specific pathogen involved. Elderly patients, particularly those over 65 years of age, face a significantly higher risk of mortality. In Spain, for example, it is estimated that 92% of pneumonia-related deaths occur in individuals over 70 years old [106]. Children under five, especially infants, are also at increased risk, with pneumonia being a leading cause of childhood mortality worldwide [5].

Comorbidities such as diabetes mellitus, heart disease, and immunosuppression due to conditions like HIV or chemotherapy further worsen the prognosis [114]. The presence of multiresistant pathogens, such as methicillin-resistant Staphylococcus aureus (MRSA) or carbapenem-resistant Enterobacterales, is associated with higher morbidity and mortality, especially in hospital-acquired pneumonia [11].

Clinical Tools for Risk Stratification

To guide treatment decisions and predict outcomes, clinicians use validated severity scores such as the Pneumonia Severity Index (PSI) and CURB-65. The PSI categorizes patients into five classes based on age, comorbidities, vital signs, and laboratory findings, helping identify those at low risk who can be safely managed as outpatients [132]. CURB-65, which assesses confusion, urea levels, respiratory rate, blood pressure, and age over 65, is simpler and widely used in emergency settings; a score of 3 or more indicates a high risk of mortality and the need for hospitalization [133].

Long-Term Outcomes and Functional Impairment

Even after recovery, some patients experience long-term consequences. Studies have shown that a significant proportion of individuals, particularly those who had severe pneumonia or were hospitalized, may have persistent functional impairments, including reduced lung function and exercise capacity [52]. In children, post-pneumonia sequelae may go unnoticed clinically but can be detected through pulmonary function tests, warranting early rehabilitation if necessary [53]. Similarly, survivors of pneumonia caused by SARS-CoV-2 often exhibit residual lung abnormalities on computed tomography (CT) scans months after infection, such as fibrosis or bronchiectasis, which may contribute to chronic respiratory symptoms [136].

Impact of Vaccination on Prognosis

Vaccination plays a critical role in improving prognosis by preventing infection altogether or reducing disease severity. The introduction of pneumococcal conjugate vaccine (PCV) has significantly reduced the incidence of invasive pneumococcal disease and severe pneumonia, particularly in children and older adults [99]. In Latin America and the Caribbean, PCV implementation has been linked to an 11% to 35% reduction in pneumonia-related mortality among children under five [138]. Annual influenza vaccination also reduces hospitalizations and complications from secondary bacterial pneumonia, especially in high-risk groups [86].

Biomarkers for Prognostic Assessment

Emerging research highlights the value of immunological biomarkers in predicting disease severity and guiding personalized treatment. The neutrophil-to-lymphocyte ratio (NLR) is a readily available marker that reflects immune dysregulation and has been associated with increased mortality in community-acquired pneumonia and viral pneumonias [140]. Elevated levels of C-reactive protein (CRP) and procalcitonin (PCT) are also strong predictors of poor outcomes and are used to guide antibiotic duration and escalation of care [141]. In critically ill patients, measurement of cytokines such as IL-6, TNF-α, and IL-8 in bronchoalveolar lavage fluid provides insight into the intensity of pulmonary inflammation and correlates with clinical severity [142].

Conclusion

The complications and prognosis of pneumonia are influenced by a complex interplay of host factors, pathogen characteristics, and healthcare access. While many cases resolve with appropriate treatment, severe forms can lead to life-threatening conditions such as septic shock, ARDS, and multiorgan failure. Risk stratification tools like PSI and CURB-65, combined with biomarker assessment, enable early identification of high-risk patients. Preventive strategies, particularly vaccination and infection control, remain essential in reducing the global burden of pneumonia and improving long-term outcomes.

Hospitalization and Critical Care Management

Hospitalization for pneumonia is indicated in cases of severe disease, high risk of complications, or when outpatient management is not feasible due to patient instability or comorbidities. Critical care management in the intensive care unit (ICU) is reserved for patients with respiratory failure, septic shock, or other life-threatening complications. The decision to admit and escalate care is guided by clinical assessment, validated prognostic scores, and the presence of specific risk factors.

Criteria for Hospitalization and ICU Admission

The decision to hospitalize a patient with pneumonia is based on a combination of clinical, laboratory, and radiological findings, along with validated severity scores. The Pneumonia Severity Index (PSI) and CURB-65 are the most widely used tools for risk stratification. The PSI categorizes patients into five classes, with Class I-II indicating low risk (mortality <1–3%) and suitability for outpatient treatment, while Class IV-V (mortality >27–31%) necessitates hospitalization, often in the ICU [132]. CURB-65, which assesses confusion, urea levels, respiratory rate, blood pressure, and age ≥65, is simpler and more commonly used in emergency settings; a score of ≥3 points indicates high risk and mandates hospitalization [133].

Clinical indicators of severity that warrant hospitalization include signs of respiratory failure (PaO₂ <60 mmHg, need for supplemental oxygen), hemodynamic instability (hypotension requiring vasopressors), altered mental status, inability to maintain hydration or oral medication, and the presence of significant comorbidities such as chronic obstructive pulmonary disease, heart failure, or immunosuppression [145]. For ICU admission, major criteria include the need for invasive mechanical ventilation or septic shock requiring vasopressors, while minor criteria (at least three required) include respiratory rate ≥30 breaths per minute, PaO₂/FiO₂ ≤250, multilobar infiltrates, uremia (BUN >20 mg/dL), and leucopenia [146].

Oxygen Therapy and Ventilatory Support

In severe pneumonia, hypoxemia is a primary concern, and oxygen therapy is essential to maintain adequate tissue oxygenation. The target saturation is 88–92% in patients at risk of hypercapnia (e.g., those with COPD) and 94–98% in others [147]. Devices range from low-flow systems (nasal cannula) to high-flow nasal oxygen (HFNC), which improves oxygenation and reduces the need for intubation [148]. Non-invasive ventilation (NIV) may be used in acute respiratory failure without contraindications, aiming to avoid intubation.

When conventional oxygen therapy fails, invasive mechanical ventilation is required, particularly if pneumonia progresses to acute respiratory distress syndrome (ARDS). A protective ventilation strategy is critical to prevent ventilator-induced lung injury (VILI). This includes using low tidal volumes (6 mL/kg of ideal body weight), limiting plateau pressure to <30 cmH₂O, and applying positive end-expiratory pressure (PEEP) to maintain alveolar recruitment [74]. FiO₂ should be titrated to the lowest level necessary to maintain adequate oxygenation, avoiding oxygen toxicity [150]. In refractory cases, advanced support such as extracorporeal membrane oxygenation (ECMO) may be considered [75].

Management of Sepsis and Hemodynamic Instability

Pneumonia is a common cause of sepsis and septic shock, requiring prompt recognition and intervention. Hemodynamic instability manifests as hypotension (systolic BP <90 mmHg), tachycardia (>120 bpm), and signs of tissue hypoperfusion such as cool extremities, prolonged capillary refill, oliguria, or lactic acidosis [152]. Immediate management includes fluid resuscitation, administration of broad-spectrum antibiotics within one hour, and vasopressors (e.g., norepinephrine) if hypotension persists despite fluids [153]. Continuous hemodynamic monitoring, including arterial line placement and advanced cardiac output measurement, allows for real-time adjustments in fluid and vasoactive therapy [154].

Prevention of Hospital-Acquired Infections

Patients in the ICU, especially those on mechanical ventilation, are at high risk for hospital-acquired infections, particularly ventilator-associated pneumonia (VAP). Prevention relies on a multifaceted approach known as the VAP prevention bundle, which includes elevating the head of the bed to 30–45° to prevent aspiration, performing oral care with chlorhexidine every 12 hours, using endotracheal tubes with subglottic suction ports, minimizing sedation, and implementing daily sedation interruption protocols to facilitate early weaning [95]. Strict hand hygiene with alcohol-based solutions and the use of personal protective equipment are essential to prevent cross-transmission of pathogens [156]. Additionally, early enteral feeding with gastric residual monitoring and proper cuff pressure management (20–30 cmH₂O) further reduce aspiration risk [157].

Monitoring for Clinical Deterioration

Close monitoring is essential to detect early signs of clinical deterioration in critically ill patients with pneumonia. Key indicators include worsening respiratory status (increased work of breathing, rising respiratory rate, persistent SpO₂ <90% despite oxygen), hemodynamic instability (hypotension, tachycardia, oliguria), and neurological changes such as confusion or decreased Glasgow Coma Scale score [1]. Laboratory markers such as rising lactate, leucocytosis or leucopenia, and elevated procalcitonin also signal disease progression or uncontrolled infection [159]. Radiological progression, such as extension of infiltrates within 72 hours despite appropriate therapy, may indicate treatment failure or complications like lung abscess or empyema and warrants reevaluation of the antimicrobial regimen and consideration of further diagnostic procedures [160].

Role of the Nurse in Critical Care and Palliative Support

The nurse plays a central role in the critical care management of pneumonia, particularly in patient education, monitoring, and coordination of care. During hospitalization, nurses educate patients and families about the diagnosis, treatment plan, and importance of adherence to therapy [161]. They are responsible for implementing VAP prevention measures, ensuring proper positioning, oral care, and ventilator circuit management [162]. In the transition to home care, nurses assess readiness for discharge using tools like PSI or CURB-65, instruct on warning signs (e.g., persistent fever, worsening dyspnea), and coordinate follow-up with primary care or home health services [163].

In cases of advanced pneumonia with poor prognosis, the nurse facilitates the shift to palliative care, focusing on symptom relief and patient dignity. Key strategies include managing dyspnea with opioids like morphine, controlling pain and agitation with appropriate medications, and providing emotional and spiritual support to both patient and family [164]. Sedation may be used for refractory symptoms, always with informed consent and ethical oversight. The nurse ensures that care aligns with patient values and advance directives, promoting a peaceful and dignified end-of-life experience [165].

Public Health and Epidemiological Surveillance

Public health surveillance for pneumonia is a critical component of global health systems, enabling the monitoring of disease burden, identification of outbreaks, and evaluation of prevention strategies. Effective surveillance relies on integrated, sentinel, and molecular systems that operate in both community and hospital settings. These systems allow for early detection of emerging pathogens, tracking of antimicrobial resistance, and assessment of the impact of vaccination programs [166]. The World Health Organization World Health Organization and regional bodies such as the Pan American Health Organization Pan American Health Organization (PAHO) play key roles in coordinating surveillance efforts across countries [167].

Surveillance Systems and Networks

The most effective surveillance strategies combine multiple approaches to provide comprehensive data. The Sentinel Surveillance of Acute Respiratory Infections (SARI) system, such as Spain’s SiVIRA network coordinated by the National Epidemiology Center (CNE) of the Instituto de Salud Carlos III (ISCIII), uses sentinel sites in primary care and hospitals to collect weekly data on influenza, SARS-CoV-2, respiratory syncytial virus (RSV), and pneumonia [168]. This system enables real-time monitoring of seasonal trends, early outbreak detection, and severity assessment. A study in La Rioja found that sentinel surveillance was more cost-effective and sufficiently representative compared to universal surveillance for detecting changes in viral circulation and pneumonia burden [169].

Hospital-based surveillance of severe acute respiratory infections (SARI) is essential for identifying cases of severe pneumonia. Protocols in regions like Andalusia and Valencia mandate reporting of severe pneumonia cases with specimen collection for virological and bacteriological diagnosis, including testing for influenza, SARS-CoV-2, RSV, and Streptococcus pneumoniae [170]. These systems help characterize etiological agents, detect nosocomial outbreaks, and evaluate control measures.

Regional networks enhance cross-border collaboration. SARInet plus and SIREVA II, promoted by PAHO, integrate respiratory virus surveillance across multiple countries, facilitating data sharing, early outbreak detection, and coordinated responses [167]. In 2026, PAHO published technical guidelines for an integrated approach to routine respiratory virus surveillance in the Americas, promoting methodological harmonization [172]. Additionally, the PAHOGen network, launched in 2023, advances regional genomic surveillance to identify emerging pathogens and variants with increased transmissibility or resistance [173].

Molecular and Genomic Surveillance

Molecular techniques are vital for detecting emerging and resistant pathogens. Multiplex real-time PCR with high-resolution melting analysis enables rapid identification of carbapenemase and colistin resistance genes in clinical isolates of Enterobacterales, such as Klebsiella pneumoniae, which can cause resistant hospital-acquired pneumonia [174]. The Centers for Disease Control and Prevention Centers for Disease Control and Prevention (CDC) provides standardized protocols for culturing and identifying resistant bacteria in clinical settings [175].

Genotyping of pathogens allows monitoring of serotype and antimicrobial resistance patterns. In Colombia, 11 years of surveillance revealed shifts in invasive serotype distribution of Streptococcus pneumoniae, impacting conjugate vaccine effectiveness [176]. In Chile, weekly genotyping of respiratory viruses, including SARS-CoV-2, tracks variants and their association with pneumonia cases [177]. Such data inform vaccine updates and public health interventions.

Antimicrobial Resistance Surveillance

Surveillance of antimicrobial resistance (AMR) is essential for guiding empirical treatment and preventing the spread of multidrug-resistant organisms. Argentina’s National Commission for Antimicrobial Resistance Control (CoNaCRA) coordinates national surveillance using WHONET, analyzing data from 2013 to 2023 [178]. Chile’s Public Health Institute publishes regular bulletins on resistance in respiratory pathogens, including Pseudomonas aeruginosa and Klebsiella pneumoniae [179]. Mexico has developed national AMR reports in hospitals, highlighting the need for continuous monitoring [180]. Pseudomonas aeruginosa is classified as a high-priority pathogen by the World Health Organization due to its resistance to multiple antibiotics, including carbapenems [68].

Notification Systems and Early Warning

In several countries, severe pneumonia is a notifiable condition. In Mexico, hospitalization-requiring pneumonia cases must be reported immediately [182]. Argentina’s National Epidemiological Bulletin includes data on acute respiratory infections and pneumonia, enabling real-time monitoring [183]. Canada’s FluWatch+ system integrates data on pneumonia hospitalizations, viral circulation, and resistance, showing significant case increases (33% in 2025), underscoring the importance of integrated surveillance [184].

Impact of Vaccination on Disease Burden

Vaccination has significantly reduced the population-level burden of pneumonia. The introduction of pneumococcal conjugate vaccines (PCVs) such as PCV10 and PCV13 has led to a 50% reduction in severe pneumococcal infections in Spain, as confirmed by the ISCIII [99]. In Latin America and the Caribbean, PCV introduction has reduced pneumonia mortality in children under five by 11% to 35%, preventing approximately 4,500 child deaths since implementation [138]. These vaccines have also altered nasopharyngeal carriage epidemiology, reducing circulation of vaccine-covered serotypes and generating herd protection in unvaccinated adults [187]. However, serotype replacement with non-vaccine types highlights the need for ongoing surveillance and higher-valency vaccines like PCV15 and PCV20 [188].

Annual influenza vaccination also plays a crucial role in pneumonia prevention. Studies show it significantly reduces hospitalization and severe complications, including pneumonia, respiratory failure, and cardiovascular events [189]. In older adults, influenza vaccination reduces mortality among hospitalized patients and decreases pneumonia incidence [86]. Combined pneumococcal and influenza vaccination in the elderly has been shown to reduce morbidity and mortality, as seen in Mexico’s 2025–2026 Winter Vaccination Campaign, which distributed over 50 million doses of vaccines against pneumococcus, influenza, and COVID-19 [101].

References