Pneumonia is a leading cause of hospitalization and death worldwide, affecting individuals across all age groups. It is an acute respiratory infection that primarily impacts the lungs leading to the filling of small air sacs or alveoli, with pus and fluid. This obstruction prevents the proper exchange of oxygen, making breathing painful and difficult.
Globally, pneumonia remains the single largest infectious cause of death in children claiming the lives of 740,180 children under the age of five in 2019 alone. This accounted for 14% of all deaths in children under five and 22% of deaths in children aged 1 to 5 years. While pneumonia is a threat to people of all ages, its impact is particularly devastating in Southern Asia and sub-Saharan Africa where healthcare access may be limited. The disease continues to burden children and families everywhere, despite the availability of effective preventive measures and treatments.
For adults, pneumonia is one of the leading causes of hospitalization. It’s responsible for tens of thousands of deaths annually in the United States alone with older adults (particularly those over 65 years) being especially vulnerable. While most pneumonia cases are treatable with antibiotics, it remains a persistent challenge, with many cases going undiagnosed or untreated. Alarmingly, only one-third of children with pneumonia receive the antibiotics they need. This article provides an in-depth understanding of pneumonia covering its causes, types, risk factors, signs, symptoms, pathophysiology and complications.
What is Pneumonia?
Pneumonia is an acute respiratory infection that affects the lungs, specifically the alveoli, which are small air sacs responsible for oxygen exchange. When a person develops pneumonia, these air sacs become inflamed and filled with fluid or pus, impairing the lung’s ability to exchange oxygen and causing breathing difficulties.1 Pneumonia can be caused by various microorganisms including bacteria, viruses and fungi, and its severity can range from mild to life-threatening depending on the causative agent, the individual’s age and underlying health conditions.2
Types of Pneumonia
Pneumonia is a diverse clinical entity and can be classified in several ways depending on factors like location of acquisition, immune status of the host, causative organisms, clinical course and anatomical patterns. Understanding these classifications helps to guide diagnosis, management and appropriate treatment.3,4
1. Classification by Location of Acquisition
This is the most widely used clinical classification and is based on where and how the pneumonia was acquired. It also gives insight into likely causative pathogens and antibiotic resistance patterns.
a. Community-Acquired Pneumonia (CAP):
● Acquired outside healthcare settings.
● Typically caused by Streptococcus pneumoniae, Haemophilus influenzae, respiratory viruses, Staphylococcus aureus, Mycoplasma pneumoniae, Klebsiella pneumoniae, Moraxella catarrhalis and Legionella pneumophila.
● Symptoms include fever, cough, chest pain and difficulty breathing.
● Disease severity depends on host factors such as age, immune status and comorbidities.
b. Hospital-Acquired Pneumonia (HAP):
● Occurs ≥48 hours after hospital admission and was not incubating at the time of admission.
● Often caused by Pseudomonas aeruginosa, Klebsiella pneumoniae, Acinetobacter species, Escherichia coli and Staphylococcus aureus.
● Risk factors include mechanical ventilation, malnutrition, prior antibiotic use and underlying diseases.
c. Ventilator-Associated Pneumonia (VAP):
● A subset of HAP developing in patients mechanically ventilated for over 48 hours.
● Typically caused by multidrug-resistant organisms and often more severe.
2. Classification by Immune Status of the Host
The immune condition of the patient plays a crucial role in determining the disease course and treatment strategy.
a. Pneumonia in Immunocompetent Individuals:
● These individuals have a normal immune response.
● Pathogens and presentations tend to be typical.
b. Pneumonia in Immunocompromised Hosts:
● Affects individuals with HIV/AIDS, malignancies, organ transplants or immunosuppressive therapies.
● Broader pathogen range includes Pneumocystis jirovecii, Cytomegalovirus (CMV), Mycobacterium avium-intracellulare, Candida and Aspergillus species.
● Presentations may be atypical or subtle.
3. Classification by Causative Microorganism
Although often not confirmed at the time of diagnosis, microbiological classification can guide targeted therapy when available.
a. Bacterial Pneumonia:
● Commonly caused by Streptococcus pneumoniae, Staphylococcus aureus, Haemophilus influenzae, Legionella pneumophila.
● Typical signs include high fever, productive cough and chest discomfort.
b. Viral Pneumonia:
● More common in children and during influenza outbreaks.
● Caused by influenza virus, RSV, adenovirus, SARS-CoV-2.
● May range from mild to severe, especially in vulnerable populations.
c. Fungal Pneumonia:
● Occurs predominantly in immunosuppressed patients.
● Pathogens include Aspergillus, Histoplasma capsulatum, Cryptococcus neoformans and Blastomyces dermatitidis.
d. Mycobacterial Pneumonia:
● Mainly due to Mycobacterium tuberculosis.
● Chronic in nature with symptoms like persistent cough, night sweats and weight loss.
e. Parasitic Pneumonia:
● Rare; often seen in tropical regions or immunosuppressed individuals.
● Caused by Toxoplasma gondii, Strongyloides stercoralis, and Paragonimus westermani.
4. Classification by Anatomical Pattern
This is based on radiological and pathological findings.
a. Lobar Pneumonia:
● Involves a large part or entire lobe of the lung.
● Often caused by Streptococcus pneumoniae, Staphylococcus aureus, H. influenzae, Klebsiella pneumoniae, Proteus, and E. coli.
● Presents with sudden fever, chest pain and dyspnea.
b. Bronchopneumonia (Lobular Pneumonia):
● Patchy inflammation centered around bronchioles affecting multiple lobules.
● Common in elderly or debilitated patients.
● Presents with productive cough, breathlessness, and scattered infiltrates on imaging.
5. Classification by Clinical Course
a. Acute Pneumonia:
● Sudden onset with rapid progression.
● Often due to common bacterial and viral agents like S. pneumoniae, M. pneumoniae and Chlamydia pneumoniae.
● Symptoms include fever, chills, fatigue and shortness of breath.
● Typically resolves in <3 weeks with appropriate treatment.
b. Chronic Pneumonia:
● Gradual onset lasting weeks to months.
● Caused by organisms like Mycobacterium tuberculosis, Nocardia, Actinomyces, Histoplasma, and Coccidioides.
● Presents with chronic cough, night sweats, weight loss and malaise.
6. Other Special Types of Pneumonia
a. Aspiration Pneumonia
● Occurs when oropharyngeal or gastric contents are inhaled into the lower airways.
● Common in patients with dysphagia, impaired consciousness or GERD.
● Anaerobic organisms and mixed flora from the oral cavity are often responsible.
● Lipid aspiration (e.g., liquid paraffin) may result in lipid pneumonia.
● Severe cases may lead to respiratory failure or death.
b. Stroke-Associated Pneumonia
● Refers to pneumonia developing within 7 days of an acute stroke.
● Caused by aspiration due to impaired swallowing and cough reflexes.
● Contributes significantly to post-stroke mortality.
c. Chemical Pneumonia:
● Results from inhalation of toxic chemicals or fumes (e.g., pesticides).
● Not infectious but induces significant pulmonary inflammation.
d. Severe Acute Respiratory Syndrome (SARS):
● Caused by SARS-CoV coronavirus.
● Characterized by tachypnea, pleuritic chest pain and hypoxia.
● Similar syndromes include COVID-19 and MERS.
e. Necrotizing Pneumonia:
● Severe form leading to lung tissue necrosis and cavitation.
● Caused by pathogens like Staphylococcus aureus, Klebsiella pneumoniae, Haemophilus influenzae, Streptococcus pyogenes, Pseudomonas, viruses (e.g., adenovirus, HIV) and fungi (e.g., Aspergillus).
● May result in lung abscess and sepsis.
Pathogenesis of Pneumonia
Pneumonia occurs when harmful microorganisms such as bacteria, viruses, fungi or parasites invade the lower respiratory tract particularly the alveoli and lung parenchyma, overcoming the lung’s innate defense mechanisms. Under normal circumstances, the lungs are home to a stable community of microbes including harmless bacteria like Streptococcus and Mycoplasma, which are kept under control by the immune system. However, certain conditions can disturb this microbial balance and weaken host defenses such as prior viral infections (like influenza) or immunosuppression caused by underlying illness, medications or treatments such as chemotherapy. Once this balance is disrupted, pathogenic microorganisms can multiply unchecked and initiate infection. Microorganisms can enter the lungs through several routes.
1. Inhalation – The most common route is inhalation, where infectious droplets are breathed in, as often happens in viral infections like the flu or common cold.
2. Aspiration – Aspiration is another route where food, liquids or gastric contents accidentally enter the lungs instead of the stomach, this is frequently seen in stroke patients, those with swallowing difficulties or unconscious individuals.
3. Hematogenous spread – Hematogenous spread involves pathogens traveling through the bloodstream from infections elsewhere in the body to the lungs.
4. Direct spread – Direct extension can occur when an infection from a nearby anatomical area such as the chest wall, spreads into the lung tissue.
Once inside, the pathogens localize in the alveoli and start replicating, leading to infection and tissue damage. The body’s immune response is rapidly activated upon recognition of these pathogens. The type of immune cells recruited depends on the nature of the invading organism:
● Neutrophils (white blood cells) respond mainly to bacteria.
● Lymphocytes target viruses.
● Granulomas (clusters of immune cells) form with fungal or TB infections.
These immune cells release various inflammatory mediators such as cytokines and prostaglandins to fight the infection. This inflammatory response causes vasodilation (widening of blood vessels) and increased vascular permeability, which allows immune cells and plasma to move into the lung tissue. As a result, fluid leaks into the alveoli causing alveolar edema. Furthermore, immune cells infiltrate the alveolar spaces and together with the fluid create areas of consolidation, regions filled with immune cells, pathogens and exudate, that appear as white patches on chest X-rays. These pathophysiological changes significantly impair the lungs’ ability to exchange gases efficiently. The accumulation of fluid and immune cells in the alveoli blocks the movement of oxygen into the bloodstream and the removal of carbon dioxide leading to clinical symptoms such as cough, shortness of breath and hypoxia (low blood oxygen levels).
In immuno-compromised individuals, the situation becomes more severe. Not only are they more susceptible to pneumonia but they are also at risk from unusual or opportunistic pathogens that rarely affect healthy people. Immune deficiencies can be broadly classified into three types:
1. Cell-mediated defects (such as T-cell dysfunction in HIV/AIDS),
2.Humoral deficiencies (like reduced antibody production seen in some genetic disorders)
3.Neutrophil dysfunction (often caused by chemotherapy or bone marrow suppression).
Understanding the nature of the patient’s immune compromise helps healthcare professionals anticipate which pathogens are likely to be involved, evaluate the severity of the infection and determine the appropriate diagnostic tests and treatment strategies.3,4
Signs and Symptoms of Pneumonia
Pneumonia symptoms can vary widely depending on age, immune status, and the severity of the infection. Symptoms may develop suddenly or gradually and range from mild to severe.5,6,7,8
Common Symptoms (in Most People)
● Cough (may produce yellow or green mucus, or be dry)
● Fever (high temperature) or chills
● Chest pain (especially when breathing or coughing)
● Shortness of breath or difficulty breathing
● Fatigue or low energy
● Loss of appetite
● Muscle aches or body pain
● Wheezing (more common in children; babies may grunt when breathing)
● Nausea, vomiting or diarrhea
● Headache
● Rapid breathing or racing pulse
Some people may not show typical symptoms like cough or fever. Instead, symptoms may be subtle or atypical.
In Older Adults
● Symptoms may be less typical or subtle
● May have confusion, drowsiness or appear suddenly weak
● Fever may be absent or replaced by a lower-than-normal temperature
● May experience worsening of underlying health conditions
In Babies and Young Children
● Fever, cough, vomiting or appearing tired/restless
● Bluish skin or lips (cyanosis)
● Grunting sounds while breathing
● Nasal flaring (widening of nostrils when breathing)
● Intercostal retractions (pulling in of chest muscles between the ribs)
● Poor feeding or low energy
Warning Signs of Secondary Bacterial Pneumonia
● A new fever or worsening symptoms following a viral infection like a cold or flu may indicate a secondary bacterial infection.
Risk Factors of Pneumonia
Pneumonia risk is influenced by multiple factors. Understanding these factors is essential for prevention and early intervention, especially in vulnerable populations.9,10,11,12
1. Age-Related Vulnerability
● Infants and Young Children (especially under 2 years): Their immune systems are still maturing, making them more prone to infections. Premature babies are at even higher risk.
● Older Adults (65 years and above): Aging naturally weakens the immune system. Seniors are also more likely to have other chronic illnesses that can exacerbate pneumonia risk. Risk increases progressively with age.
2. Environmental and Occupational Exposure
● Crowded Living Conditions: Frequent exposure in places like prisons, nursing homes, shelters and military barracks increases transmission risk.
● Air Quality: Regular inhalation of air pollutants or toxic chemicals (e.g., from industrial work) increases respiratory vulnerability.
● Animal Contact: Occupations involving birds or animals (e.g., poultry processing, pet shops, veterinary clinics) expose individuals to zoonotic pathogens.
3. Lifestyle and Behavioral Factors
● Smoking: Impairs the lungs’ natural defense mechanisms reducing the ability to clear mucus and pathogens.
● Excessive Alcohol Use and Drug Abuse: Weakens immunity and raises the risk of aspiration (inhaling food or vomit into the lungs), especially when sedated or unconscious.
● Exposure to Sick Individuals: Close contact with infected persons increases risk, particularly in unvaccinated individuals.
4. Medical Conditions and Immune Status
● Neurological Disorders: Conditions like stroke, dementia and Parkinson’s disease may affect swallowing and coughing, increasing the likelihood of aspiration pneumonia.
● Weakened Immune System:
○ Due to HIV/AIDS, cancer treatments, organ transplants,or long-term use of steroids or immunosuppressive drugs.
○ Pregnancy and malnutrition (especially in non-breastfed infants) also contribute to immune compromise.
● Chronic Illnesses: Several long-term conditions are associated with higher pneumonia risk including:
○ Chronic obstructive pulmonary disease (COPD), asthma, cystic fibrosis
○ Diabetes mellitus
○ Heart failure and cardiovascular diseases
○ Liver disease and kidney disease
○ Sickle cell disease
5. Hospital-Related Factors
● Hospitalization and ICU Admission: Immobile or unconscious patients are at higher risk for hospital-acquired pneumonia.
● Mechanical Ventilation: Increases the risk of ventilator-associated pneumonia, a serious form of the disease.
6. Seasonal Trends
● Although pneumonia can occur year-round, fall and winter seasons see increased respiratory infections, making pneumonia more common during these times.
7. Pediatric-Specific Risk Factors
In addition to age, children’s risk increases with:
● Indoor air pollution from biomass fuels (wood, dung)
● Living in overcrowded homes
● Parental smoking
● Previous illnesses like measles or HIV
Complications of Pneumonia
Pneumonia can cause serious problems, especially in older adults, people with other illnesses, and hospital-acquired cases. About 53% of pneumonia patients may have complications.13,14,15,16
1. Respiratory Complications:
● Pleural effusion (fluid buildup around the lungs)
● Parapneumonic effusion (fluid around lungs due to pneumonia) and empyema (pus in the fluid around lungs)
● ARDS (Acute Respiratory Distress Syndrome) (severe lung inflammation making breathing difficult)
● Respiratory failure (lungs can’t provide enough oxygen or remove carbon dioxide)
● Necrotizing pneumonia (lung tissue starts dying due to infection)
● Pneumatoceles (air-filled cysts in the lungs)
● Multilobar consolidation (pneumonia affecting multiple parts of the lungs)
2. Sepsis and Septic Shock:
● Sepsis (serious infection spreading through the blood)
● Septic shock (dangerously low blood pressure from infection)
3. Cardiac Complications:
● Heart failure (heart can’t pump blood properly)
● Arrhythmias (irregular heartbeats)
● Myocardial infarction (heart attack)
4. Neurological Complication:
● Meningitis (infection of the brain and spinal cord lining)
5. Liver/Bile Complication:
● Cholestatic jaundice (bile flow is blocked, causing yellowing of skin/eyes)
Conclusion
Pneumonia remains a significant global health concern particularly affecting the most vulnerable populations such as young children, the elderly and individuals with weakened immune systems. Understanding its diverse causes from bacterial and viral to fungal pathogens and recognizing the different types and associated risk factors is vital for early identification and prevention.
The hallmark symptoms ranging from cough and chest pain to fever and difficulty breathing, reflect the underlying inflammatory and immune responses occurring within the lungs. Left untreated, pneumonia can lead to serious complications including respiratory failure, sepsis and lasting lung damage. This comprehensive exploration of pneumonia’s origins, mechanisms and manifestations lays the foundation for informed awareness.
In the next article, we will delve into the crucial aspects of diagnosis and treatment, equipping readers with the knowledge to recognize the condition early and understand the strategies that can lead to effective recovery and prevention.
References
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- Torres, A., Cilloniz, C., Niederman, M.S. et al. Pneumonia. Nat Rev Dis Primers 7, 25 (2021). https://www.nature.com/articles/s41572-021-00259-0#citeas
- Lim WS. Pneumonia—Overview. Encyclopedia of Respiratory Medicine. 2022:185–97. https://pmc.ncbi.nlm.nih.gov/articles/PMC7241411/
- Krushna K. Zambare, Avinash B. Thalkari. Overview on Pathophysiology of Pneumonia. Asian J. Pharm. Res. 2019; 9(3):177-180. https://asianjpr.com/HTMLPaper.aspx?Journal=Asian%20Journal%20of%20Pharmaceutical%20Research;PID=2019-9-3-7
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- American Thoracic Society. What is pneumonia. Patient Education:Information Series. Am J Respir Crit Care Med Vol. 193, P1-P2, 2016. Online version updated October 2020. https://www.thoracic.org/patients/patient-resources/resources/what-is-pneumonia.pdf
- National Heart, Lung and Blood Institute. Pneumonia; Symptoms. Accessed on May 07, 2025. Available from: https://www.nhlbi.nih.gov/health/pneumonia/symptoms
- org [Internet]. Cologne, Germany: Institute for Quality and Efficiency in Health Care (IQWiG); 2006-. Overview: Pneumonia. [Updated 2021 May 18]. Available from: https://www.ncbi.nlm.nih.gov/books/NBK525774/
- National Heart, Lung and Blood Institute. Pneumonia; Causes and Risk Factors. Accessed on May 07, 2025. Available from: https://www.nhlbi.nih.gov/health/pneumonia/causes
- Risk Factors for Pneumonia. CDC. Published on October 17, 2023. Accessed on May 07, 2025. Available from: https://www.cdc.gov/pneumonia/risk-factors/index.html
- Pneumonia in Children. WHO. Published on November 11, 2022. Accessed on May 07, 2025. Available from: https://www.who.int/news-room/fact-sheets/detail/pneumonia
- Torres A, Peetermans WE, Viegi G, Blasi F. Risk factors for community-acquired pneumonia in adults in Europe: a literature review. Thorax. 2013 Nov;68(11):1057-65. https://pmc.ncbi.nlm.nih.gov/articles/PMC3812874/
- Alshahwan SI, Alsowailmi G, Alsahli A, Alotaibi A, Alshaikh M, Almajed M, Omair A, Almodaimegh H. The prevalence of complications of pneumonia among adults admitted to a tertiary care center in Riyadh from 2010-2017. Ann Saudi Med. 2019 Jan-Feb;39(1):29-36. https://pmc.ncbi.nlm.nih.gov/articles/PMC6464674/#sec13
- Mani CS. Acute Pneumonia and Its Complications. Principles and Practice of Pediatric Infectious Diseases. 2018:238–249.e4. https://pmc.ncbi.nlm.nih.gov/articles/PMC7173499/#s0160
- Cillóniz, C. et al. Pulmonary complications of pneumococcal community-acquired pneumonia: incidence, predictors, and outcomes. Clinical Microbiology and Infection.2012;18(11):1134 – 1142. https://www.clinicalmicrobiologyandinfection.com/article/S1198-743X(14)60751-5/fulltext
- Vicente F. Corrales-Medina et al. Cardiac Complications in Patients With Community-Acquired Pneumonia: Incidence, Timing, Risk Factors, and Association With Short-Term Mortality. Circulation. 2012;125: 6. https://www.ahajournals.org/doi/10.1161/circulationaha.111.040766
