Pneumonia in people whose immune system is weakened (for example, by AIDS, organ transplantation, or the use of certain drugs) is usually caused by different organisms than those that cause pneumonia in healthy people.
Pneumocystis jiroveci is a common fungus that may reside harmlessly in the lungs of healthy people. It usually causes pneumonia only when the body's defenses are weakened because of cancer, drugs that alter the immune system, or AIDS. Drugs that alter the immune system include corticosteroids, chemotherapy drugs, and drugs used to treat autoimmune disorders. Often, P. jiroveci pneumonia is the first indication that a person with human immunodeficiency virus (HIV) infection has developed AIDS.
Most people develop a fever, shortness of breath, and a dry cough. These symptoms usually arise over several weeks. The lungs may not be able to deliver sufficient oxygen to the blood, leading to shortness of breath that is sometimes severe.
X-rays show either no abnormality or patchy infection, similar to that which occurs in some viral infections. The diagnosis is made by microscopic examination of expectorated sputum or from sputum obtained by induction (in which a vapor is used to stimulate coughing) or bronchoscopy (in which an instrument is inserted into the airways to collect a specimen—see Diagnosis of Lung Disorders: Bronchoscopy).
The combination antibiotic trimethoprim-sulfamethoxazole can be used to help prevent Pneumocystis pneumonia in people at risk. This drug's side effects, which are particularly common in people who have AIDS, include rashes, a reduced number of infection-fighting white blood cells, and fever. Alternative preventive drug treatments are dapsone, atovaquone, and pentamidine (which can be taken as an aerosol, inhaled directly into the lungs).
Drugs used to treat Pneumocystis pneumonia are trimethoprim-sulfamethoxazole, dapsone combined with trimethoprim, clindamycin and primaquine, atovaquone, or intravenous pentamidine. When the level of oxygen in the blood falls below a certain level, corticosteroids may also be given.
Even when the pneumonia is treated, the overall death rate is 15 to 20%.
Last full review/revision April 2008 by John G. Bartlett, MD