PNEUMONIA CAUSES

The list of organisms which can cause pneumonia is very large, and includes nearly every class of infecting organism: viruses, bacteria, bacteria-like organisms, fungi, and parasites (including certain worms). Different organisms are more frequently encountered by different age groups. Further, other characteristics of an individual may place him or her at greater risk for infection by particular types of organisms:

• Viruses cause the majority of pneumonias in young children (especially respiratory syncytial virus, parainfluenza and influenza viruses, and adenovirus).
• Adults are more frequently infected with bacteria (such as Streptococcus pneumoniae, Haemophilus influenzae, and Staphylococcus aureus).
• Pneumonia in older children and young adults is often caused by the bacteria-like Mycoplasma pneumoniae (the cause of what is often referred to as "walking" pneumonia).
• Pneumocystis carinii is an extremely important cause of pneumonia in patients with immune problems (such as patients being treated for cancer with chemotherapy, or patients with AIDS. Classically considered a parasite, it appears to be more related to fungi.
• People who have reason to come into contact with bird droppings, such as poultry workers, are at risk for pneumonia caused by the organism Chlamydia psittaci.
• A very large, serious outbreak of pneumonia occurred in 1976, when many people attending an American Legion convention were infected by a previously unknown organism. Subsequently named Legionella pneumophila, it causes what is now called "Legionnaire's disease." The organism was traced to air conditioning units in the convention's hotel.