Which pathogen is responsible for the majority of community-acquired pneumonia cases?

Streptococcus pneumoniae is recognized as the leading cause of community-acquired pneumonia. This bacterium is responsible for a significant proportion of pneumonia cases due to its prevalence and virulence. It is a gram-positive cocci that can be found in the nasopharynx of healthy individuals without causing illness. However, under certain conditions, such as immunocompromised states or when the host defenses are weakened, it can invade the alveoli and elicit a strong respiratory response leading to pneumonia.

Streptococcus pneumoniae's ability to produce pneumolysin, a toxin that damages host cells, and its polysaccharide capsule, which helps it evade phagocytosis, are key factors in its pathogenicity. Furthermore, the widespread availability and effectiveness of vaccines, such as the pneumococcal conjugate vaccine, have addressed this pathogen, although it still remains a common cause in unvaccinated populations and certain age groups.

While Mycoplasma pneumoniae, Legionella pneumophila, and Haemophilus influenzae can also cause community-acquired pneumonia, they are less common when compared to Streptococcus pneumoniae. Mycoplasma pneumoniae is often linked to atypical pneumonia, particularly in younger populations, and