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 pneumonia often occurs in people who have a weakened immune system.
• People have shortness of breath, a dry cough, and often fever.
• X-rays of the chest are not as helpful as microscopic examinations of sputum samples for making the diagnosis.
• Trimethoprim-sulfamethoxazoleSome Trade Names
  is often used to treat this pneumonia.

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-sulfamethoxazoleSome Trade Names
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 dapsoneSome Trade Names
ACZONE
, atovaquoneSome Trade Names
MEPRON
, and pentamidineSome Trade Names
NEBUPENT
(which can be taken as an aerosol, inhaled directly into the lungs).

Drugs used to treat Pneumocystis pneumonia are trimethoprim-sulfamethoxazoleSome Trade Names
, dapsoneSome Trade Names
ACZONE
combined with trimethoprim, clindamycinSome Trade Names
CLEOCIN
and primaquine, atovaquoneSome Trade Names
MEPRON
, or intravenous pentamidineSome Trade Names
NEBUPENT
. 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