Bacterial pneumonia is caused by a pathogenic infection of the lungs and may present as a primary disease process or as the final coup de grace in the individual who is already debilitated. Bacteria from the upper airways or, less commonly, from hematogenous spread, find their way to the lung parenchyma. Once there, a combination of factors (including virulence of the infecting organism, status of the local defenses, and overall health of the patient) may lead to bacterial pneumonia. The clinical presentation varies from the mildly to extremely ill patients.