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

Streptococcus pneumoniae is recognized as the most common cause of community-acquired pneumonia (CAP). This bacterium is a gram-positive cocci that is a part of the normal flora in the upper respiratory tract of many individuals. Under certain conditions, it can invade the lungs and lead to pneumonia, particularly in populations such as the very young, elderly, or immunocompromised individuals.

The prevalence of Streptococcus pneumoniae as a pathogen in CAP is supported by numerous epidemiological studies, which consistently demonstrate its dominance over other bacteria in this context. This organism can cause severe pneumonia, presenting with typical symptoms like fever, cough, and pleuritic chest pain, along with physical examination findings of lobar consolidation.

In contrast, while other organisms such as Escherichia coli, Haemophilus influenzae, and Mycoplasma pneumoniae can also cause pneumonia, they are not as frequently implicated in community-acquired cases. Patients with pneumonia due to these organisms might present with different clinical features or risk factors. For example, E. coli is more commonly associated with healthcare-acquired infections, while Mycoplasma pneumoniae typically causes atypical pneumonia, which is often characterized by a more gradual onset and milder symptoms.