Most common cause of community-acquired pneumonia in adults?

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Streptococcus pneumoniae is the most common cause of community-acquired pneumonia in adults. It is a gram-positive, lancet-shaped diplococcus that often presents with sudden fever and a productive cough, sometimes with rusty sputum, and it typically causes focal, lobar consolidation on imaging. Its prominence comes from its widespread ability to colonize the nasopharynx and invade the lower respiratory tract across a broad adult population, including those with chronic illnesses or older age.

Other organisms have roles in CAP, but their prominence varies by patient group: Mycoplasma pneumoniae commonly causes atypical pneumonia in younger adults or milder cases; Haemophilus influenzae is more frequent in smokers or people with COPD; Legionella pneumophila is important in certain outbreaks or exposure scenarios but is less common overall. The overall epidemiology makes Streptococcus pneumoniae the leading etiologic agent in adults with community-acquired pneumonia.