Etiology

• Typical pneumonia

  • Streptococcus pneumoniae (most common)

    • Also the most common pathogen in nursing home residents

    • Most common cause of pneumonia in persons who inject drugs [1]

  • Haemophilus influenzae

  • Moraxella catarrhalis

  • Klebsiella pneumoniae

  • Staphylococcus aureus

• Atypical pneumonia

  • Bacteria

    • Mycoplasma pneumoniae (most common in the ambulatory setting)

    • Chlamydophila pneumoniae

    • Chlamydophila psittaci

    • Legionella pneumophila → legionellosis

    • Coxiella burnetii → Q fever

    • Francisella tularensis → tularemia

  • Viruses

    • RSV

    • Influenza viruses, Parainfluenza viruses

    • CMV

    • Adenovirus

    • Coronaviridae (e.g., SARS-CoV-2)

• Gram-negative pathogens

  • Pseudomonas aeruginosa

  • Enterobacteriaceae

  • Acinetobacter spp

• Staphylococci (Staphylococcus aureus)

• Streptococcus pneumoniae

For atypical pneumonia bacterial causes, remember the mnemonic: Atypically, Legions of Clams Mind their P's and Q's!• Legionella pneumophila• Chlamydophila pneumoniae• Mycoplasma pneumoniae• Psittacosis (Chlamydophila psittaci)• Q fever (Coxiella burnetii)

• Old age and immobility of any cause

• Chronic diseases

  • Preexisting cardiopulmonary conditions (e.g., bronchial asthma, COPD, heart failure)

  • Acquired or congenital abnormalities of the airways (e.g., bronchiectasis, space-occupying lesions, cystic fibrosis)

• Immunosuppression

  • HIV infection

  • Diabetes mellitus

  • Cytostatic and immunosuppressive therapy

  • Alcoholism

  • Malnutrition

• Impaired airway protection

  • Alteration in consciousness (e.g., due to stroke, seizure, anesthesia, drugs, alcohol)

  • Dysphagia

  • Smoking

• Environmental factors

  • Crowded living conditions (e.g., prisons, homeless shelters)

  • Toxins (e.g., solvents, gasoline)

• Cryptogenic organizing pneumonia

  • Specific medications (e.g., amiodarone, bleomycin)

  • Chronic inflammatory disorders (e.g., rheumatoid arthritis)

• Surgical procedures

  • Upper abdominal surgery

  • Chest surgery