The etiological pathogens involved in CAP can be identified according to the disease severity. In general, patients treated in the community with no predisposing co-morbidities are more likely to have infection caused by S. pneumoniae, M. pneumoniae, C. pneumoniae and H. influenzae. However as the disease severity progresses the mortality rises and other pathogens are increasingly implicated. Enteric Gram-negative bacteria, Legionella spp. and anaerobes become problematic, and in ICU patients Pseudomonas aeruginosa and Staphylococcus aureus need to be managed effectively.