What is the most common cause of community-acquired pneumonia?

Streptococcus pneumoniae is recognized as the most common cause of community-acquired pneumonia (CAP). This Gram-positive bacterium is responsible for a significant number of pneumonia cases, particularly in adults. It is often found as part of the normal flora in the nasopharynx and can cause infection when it invades lung tissue, especially when host defenses are impaired, such as in immunocompromised individuals or the elderly.

In terms of prevalence, Streptococcus pneumoniae accounts for about 30-50% of all community-acquired pneumonia cases. Its ability to cause severe disease is attributed to its polysaccharide capsule, which helps evade phagocytosis, and its capacity to produce various virulence factors that damage host tissues.

While other pathogens, like Mycoplasma pneumoniae and Haemophilus influenzae, can also cause CAP, they are less common than Streptococcus pneumoniae. Mycoplasma pneumoniae typically leads to atypical pneumonia, which is often seen in younger populations, while Haemophilus influenzae is more frequently a concern in individuals with underlying lung disease or those who are splenectomized. Legionella pneumophila is more associated with specific outbreaks and conditions such as immunocom