Common LRT infection characterized by inflammation of the lung tissue

Aetiology

Common causative organisms

• Streptococcus pneumoniae (70%) - most common, community acquired
• Haemophilus influenza (5%) - usually the elderly and patients with COPD
• Staph. aureus (4%) - common in PWIDs, often following influenza

Atypicals

• Legionella - inhalation of contaminated water droplets
• Mycoplasma pneumonia - children and young adults, peaks every 4 years
• Coxiella burneti (Q fever) - farming
• Chlamydophila psittaci - birds (pets)
• Klebsiella - aspiration (e.g. from alcoholism)
• Pneumocytis jiroveci - immunocompromised e.g. AIDS patients

Pathophysiology

• Infection leads to fluid and blood cells leaking into the alveoli
• The infection spreads across the alveoli and eventually the lung tissue becomes consolidated, impairing the gas exchange due to reduced ventilation

Lobar pneumonia

• Confluent consolidation involving a complete lung lobe
• Most commonly Streptococcus pneumoniae (community acquired), classically in otherwise healthy young adults