The most common cause of pneumonia for each age group

Neonates (0–28 days)

• Group B Streptococcus (GBS):
  • Acquired during birth from maternal vaginal flora.
  • Neonates have immature immune defenses.
• Escherichia coli:
  • Also transmitted during delivery.
  • Common cause of early-onset neonatal sepsis and pneumonia.
• Respiratory Syncytial Virus (RSV):
  • Leading viral cause in neonates.
  • Narrow airways and immature immune system increase severity.

Infants (1 month–1 year)

• Streptococcus pneumoniae:
  • Encapsulated; vulnerable due to immature humoral immunity.
• Chlamydia trachomatis:
  • Vertical transmission during childbirth.
  • Causes afebrile pneumonia with eosinophilia at 4-12 weeks of age.
• RSV and Parainfluenza virus:
  • Airway size and underdeveloped mucosal immunity predispose to bronchiolitis and pneumonia.

Children (1–12 years)

• Streptococcus pneumoniae:
  • Still the most common bacterial cause.
  • Colonizes the nasopharynx and spreads to lungs.
• Haemophilus influenzae:
  • More common in unvaccinated children.
  • Causes lobar or bronchopneumonia.
• RSV and Parainfluenza virus:
  • Continue to be major respiratory pathogens in school-aged children.

Young Adults (12–40 years)

• Streptococcus pneumoniae:
  • Still a frequent cause of community-acquired pneumonia.
• Mycoplasma pneumoniae:
  • Causes “walking pneumonia.”
  • Spreads easily in close-contact settings (e.g., schools, military).
• Chlamydia pneumoniae:
  • Mild pneumonia and bronchitis in adolescents and young adults.
• Various respiratory viruses (e.g., adenovirus):
  • Spread rapidly in communal environments.

Older Adults (>40–50 years)

• Streptococcus pneumoniae:
  • Leading cause due to immunosenescence and comorbidities.
• Haemophilus influenzae:
  • Common in patients with COPD or smoking history.
• Legionella pneumophila:
  • Associated with contaminated water systems (e.g., air conditioning).
  • Affects smokers, elderly, and immunocompromised patients.
• Influenza virus:
  • Major cause of viral pneumonia in the elderly.
  • Can predispose to secondary bacterial pneumonia.