Cause
Pneumonia is primarily due to infections caused by bacteria or viruses and less commonly by fungi and parasites. Although there are more than 100 strains of infectious agents identified, only a few are responsible for the majority of the cases. Mixed infections with both viruses and bacteria may occur in up to 45% of infections in children and 15% of infections in adults. A causative agent may not be isolated in approximately half of cases despite careful testing.
The term pneumonia is sometimes more broadly applied to any condition resulting in inflammation of the lungs (caused for example by autoimmune diseases, chemical burns or drug reactions); however, this inflammation is more accurately referred to as pneumonitis. Infective agents were historically divided into "typical" and "atypical" based on their presumed presentations, but the evidence has not supported this distinction, thus it is no longer emphasized.
Conditions and risk factors that predispose to pneumonia include: smoking, immunodeficiency, alcoholism, chronic obstructive pulmonary disease, chronic kidney disease, and liver disease. The use of acid-suppressing medications -such as proton-pump inhibitors or H2 blockers- is associated with an increased risk of pneumonia. Old age also predisposes pneumonia.
Read more about this topic: Pneumonia