What is the most common etiological agent of community-acquired pneumonia (CAP)?

Streptococcus pneumoniae is recognized as the leading cause of community-acquired pneumonia (CAP). This bacterium is a gram-positive, encapsulated diplococcus that is highly prevalent in the general population, making it the most common etiological agent associated with pneumonia acquired outside of healthcare settings. In various studies and epidemiological reports, Streptococcus pneumoniae has been consistently identified as responsible for a significant proportion of CAP cases, especially among older adults and those with underlying health conditions.

The virulence factors of Streptococcus pneumoniae, including its polysaccharide capsule and surface proteins, contribute to its ability to evade the host's immune response, allowing for effective infection within the lung. This capsular production enhances its pathogenicity and is why vaccination against pneumococcal pneumonia is an essential public health measure.

Other organisms, like Haemophilus influenzae and Mycoplasma pneumoniae, may also contribute to cases of CAP but are seen less frequently as primary causative agents when compared to Streptococcus pneumoniae. Legionella pneumophila is typically associated with a specific type of pneumonia (Legionnaires' disease) and is less prevalent in general CAP cases. Thus, understanding the frequency and clinical impact of each of these organisms is