• Bacterial pneumonia
• Viral pneumonia
• Aspiration pneumonia
• Pneumocystis pneumonia (PCP)
• Bronchitis
• Lung abscess
• Tuberculosis
• Pulmonary embolism
• Myocardial infarction
• Sarcoidosis
• Lung neoplasm
• Hypersensitivity pneumonitis
• Bronchiolitis or bronchiolitis obliterans with organizing pneumonia (BOOP)

• Hospital-acquired pneumonia

Bacterial

• Streptococcus pneumoniae (most common)
• Haemophilus influenzae
• Mycoplasma pneumoniae (young adults, summer & fall)
• Chlamydia pneumoniae (young adults)
• Staphylococcus aureus (influenza epidemics, IV drug use, bronchiectasis)
• Neisseria meningitides
• Moraxella catarrhalis (lung disease, elderly)
• Klebsiella pneumoniae (alcohol, diabetes)
• Legionella spp. (contaminated construction site, water source, or air conditioner)

Viral

• Influenza virus
• Respiratory syncytial virus
• Adenovirus
• Parainfluenza virus

Special risk factors

• Chlamydia psittaci (psittacosis): birds
• Coxiella burnetii (Q fever): cattle, sheep, goats; especially newborns or products of conception
• Francisella tularensis (tularemia): tick or skinning rabbit
• Endemic fungi (Blastomyces: Ohio & Mississippi River valleys; Coccidioides: Southwest United States or Latin America; Histoplasma: Ohio & Mississippi River valleys)
• Sin nombre virus (hantavirus pulmonary syndrome): rodent waste, Southwest United States