Community acquired pneumonia is most commonly caused by: September 2005 and March 2008

Ans. B: Streptococcus pneumoniae The term community-acquired pneumonia is usually reserved for people who have pneumonia caused by one of the more common bacteria or viruses. Streptococcus pneumonia is the most common cause of community-acquired acute pneumonia. Gram-negative rods (Enterobacteriaceae and Pseudomonas species) and S.aureus are the most common isolates in hospitalacquuired pneumonia; unlike community-acquired pneumonias, S.pneumoniae is not a major pathogen in hospitalacquuired pneumonia. Haemophilus influenzae, Chlamydia pneumoniae, and Mycobacterium pneumoniae are the other common bacterial causes. Common viral causes include respiratory syncytial virus (RSV), adenoviruses, influenza A and B viruses, metapneumovirus, and parainfluenza viruses.