Final answer:
Streptococcus pneumoniae is the primary cause of community-acquired pneumonia, responsible for inflaming the alveoli and leading to symptoms like a productive cough with bloody sputum.
Step-by-step explanation:
The organism most commonly responsible for community-acquired pneumonia is Streptococcus pneumoniae, a gram-positive bacterium frequently found in the normal microbiota of the respiratory tract. This pathogen colonizes the bronchioles and can spread to the alveoli, where its polysaccharide capsule and other virulence factors, such as pneumolysin O, contribute to the progression of disease by damaging host cells and promoting inflammation. This inflammation causes alveoli to fill with neutrophils and red blood cells, leading to symptoms such as a productive cough with bloody sputum. While S. pneumoniae is a primary cause, other potential agents include viruses, Mycoplasma pneumoniae (leading to walking pneumonia), and, in healthcare settings, opportunistic bacteria like Klebsiella pneumoniae and Staphylococcus aureus.