Various. Causes of pneumonia

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Causes of Pneumonia

Pneumonia has a wide range of infectious and non-infectious causes, best categorized by the setting in which it is acquired and the type of pathogen.

Classification by Setting

1. Community-Acquired Pneumonia (CAP)

Onset in the community or within 48 hours of hospitalization. The most studied form.

2. Hospital-Acquired Pneumonia (HAP)

Onset >48 hours after admission to hospital, not incubating at admission.

3. Ventilator-Associated Pneumonia (VAP)

Onset ≥48 hours after mechanical ventilation. (Note: the older "healthcare-associated pneumonia" [HCAP] category has been discontinued as it did not reliably predict resistant pathogens.)

4. Aspiration Pneumonia

Accounts for 5-15% of CAP cases, caused by macroaspiration of oropharyngeal or gastric contents.

Microbial Causes of CAP (by Site of Care)

From Harrison's Principles of Internal Medicine (22nd ed., 2025):

Outpatients	Hospitalized (Non-ICU)	ICU
Streptococcus pneumoniae	S. pneumoniae	S. pneumoniae
Mycoplasma pneumoniae	M. pneumoniae	Staphylococcus aureus
Haemophilus influenzae	Chlamydia pneumoniae	Legionella spp.
Chlamydia pneumoniae	H. influenzae	Gram-negative bacilli
Respiratory viruses	Legionella spp.	H. influenzae
	Respiratory viruses	Respiratory viruses

Major Bacterial Causes

Typical Bacteria

Streptococcus pneumoniae (pneumococcus) - The most frequent cause of CAP requiring hospitalization, ~200 cases per 100,000/year in the U.S. Risk factors: smoking, HIV, alcoholism, chronic lung disease, asplenia, old age.
Haemophilus influenzae - Both typeable (type b in unimmunized) and non-typeable strains. Important in COPD patients. Mortality 2-7%, higher in bacteremic cases.
Staphylococcus aureus (including MRSA) - More common in ICU settings; an important cause of secondary bacterial pneumonia post-influenza.
Klebsiella pneumoniae - A gram-negative bacillus; classically associated with alcoholism. Can cause severe lobar pneumonia with "currant jelly" sputum.
Pseudomonas aeruginosa - Mainly HAP/VAP; CAP in patients with structural lung disease (bronchiectasis, cystic fibrosis) or immunocompromise.
Escherichia coli, other Enterobacteriaceae - More common in HAP/VAP; risk rises with healthcare exposure.

Atypical Bacteria

These often produce a "walking pneumonia" picture with gradual onset, dry cough, and extrapulmonary features:

Mycoplasma pneumoniae - Very common in school-age children and young adults; accounts for ~17% of CAP in children 5-13 years.
Chlamydophila (Chlamydia) pneumoniae - Common in adults; responsible for community outbreaks.
Legionella pneumophila - Waterborne; associated with air conditioning and cooling towers. Presents with high fever, GI symptoms, hyponatremia. Important in severe/ICU CAP.
Chlamydia trachomatis - A perinatal cause in neonates of untreated mothers.

Less Common / Unusual Bacteria

Acinetobacter baumannii - Rare in CAP; more common in HAP/VAP. CAP cases cluster in Southeast Asia and hot climates; associated with alcoholism, COPD, diabetes. Mortality can exceed 50%.
Anaerobic bacteria - Classic cause of aspiration pneumonia (e.g., Bacteroides, Fusobacterium, Peptostreptococcus). Typically polymicrobial.
Bordetella pertussis - Causes pertussis ("whooping cough") with bronchopneumonia; a consideration in children and adolescents even in the vaccinated (vaccine immunity wanes).
Listeria monocytogenes - Neonatal pneumonia with perinatal acquisition.
Group B Streptococcus - Major cause in neonates (0-30 days).
Moraxella catarrhalis - More common in COPD exacerbations.

Viral Causes

Viruses are now the most frequently identified pathogens in CAP via molecular diagnostics - detected in 27% of adult and 61-66% of pediatric CAP cases (Murray & Nadel's Respiratory Medicine).

Virus	Notes
Influenza A and B	Most common viral cause of severe adult CAP; can cause primary viral pneumonia or predispose to secondary bacterial pneumonia (especially S. pneumoniae, S. aureus)
Respiratory Syncytial Virus (RSV)	Dominant in infants <2 years; also increasingly recognized in elderly and immunocompromised adults
SARS-CoV-2 (COVID-19)	Pandemic pathogen; has significantly altered the etiologic profile of CAP
Human Metapneumovirus (hMPV)	Common in children and elderly
Parainfluenza viruses	Especially in young children
Rhinovirus	Very common; often co-detected with bacteria
Adenovirus	Can cause severe pneumonia, especially in immunocompromised
SARS-CoV-1 / MERS-CoV	Epidemic coronaviruses
Varicella-zoster (VZV)	Varicella pneumonia in adults; rare with widespread vaccination
Measles	Rare in vaccinated populations

Fungal Causes

Pneumocystis jirovecii (PCP) - Leading opportunistic pneumonia in HIV/AIDS and other immunocompromised patients. Despite the name, it is a fungus radiologically producing bilateral ground-glass opacities.
Histoplasma capsulatum - Endemic in Mississippi and Ohio River valleys; causes a range from mild flu-like illness to severe pneumonia.
Coccidioides immitis - Endemic in southwestern U.S. (San Joaquin Valley); can cause acute or chronic pneumonia.
Aspergillus spp. - Invasive in neutropenic patients; may cause necrotizing pneumonia.
Cryptococcus neoformans - Mainly in immunocompromised; causes subacute pneumonia.
Blastomyces dermatitidis - Endemic in North America; can mimic bacterial pneumonia.

Age-Specific Causes

From Tintinalli's Emergency Medicine:

Age Group	Key Pathogens
Neonates (0-30 days)	Group B Strep, E. coli, Klebsiella, Listeria, S. aureus, C. trachomatis
Infants/Toddlers (1 month - 2 years)	RSV, rhinovirus, influenza, parainfluenza, hMPV, adenovirus; S. pneumoniae (main bacterial cause)
Children (2-5 years)	Predominantly viral (RSV, rhinovirus, hMPV); S. pneumoniae, H. influenzae
School-age (5-13 years)	Viruses still predominate; M. pneumoniae rises to ~17%; C. pneumoniae, S. pneumoniae
Adolescents/Adults	S. pneumoniae, M. pneumoniae, C. pneumoniae, H. influenzae, respiratory viruses
Elderly/Hospitalized	S. pneumoniae, gram-negative bacilli, S. aureus, anaerobes, aspiration

Non-Infectious Causes

Not all pneumonia-like presentations are infectious. The differential includes:

Chemical/aspiration pneumonitis (Mendelson syndrome) - acid aspiration without bacterial infection
Hypersensitivity pneumonitis (extrinsic allergic alveolitis) - antigenic exposure (molds, bird droppings)
Organizing pneumonia (cryptogenic or secondary to drugs/connective tissue disease)
Radiation pneumonitis - post-radiotherapy
Drug-induced pneumonitis - amiodarone, methotrexate, nitrofurantoin, bleomycin
Eosinophilic pneumonia - idiopathic or drug/parasite-associated
Vasculitis (e.g., granulomatosis with polyangiitis, Goodpasture syndrome)

How Pathogens Reach the Lung

The main routes of microbial entry into the lower respiratory tract (Harrison's, 22nd ed.) are:

Microaspiration of oropharyngeal secretions (the dominant mechanism in adults; 50% of normal people aspirate small volumes during sleep)
Inhalation of airborne droplets
Direct mucosal dispersion along respiratory mucosa
Hematogenous seeding (less common; seen in endocarditis, IV drug use)

Sources:

Murray & Nadel's Textbook of Respiratory Medicine, Chapter 46 (Major/Common/Less Common Causes of Pneumonia)
Harrison's Principles of Internal Medicine, 22nd Edition (2025), Chapter 131 (CAP Etiology Table)
Tintinalli's Emergency Medicine: A Comprehensive Study, Chapter on Age-Specific Causes of Pneumonia

What are the clinical features that help differentiate bacterial from viral pneumonia?

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