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Pneumocystis pneumonia

Histopathology showing Pneumocystis cysts in the lung of a patient with AIDS

Histopathology showing Pneumocystis cysts in the lung of a patient with AIDS

Pneumocystis pneumonia (PCP) is a serious illness caused by the fungus Pneumocystis jirovecii. PCP is one of the most frequent and severe opportunistic infections in people with weakened immune systems, particularly people with HIV/AIDS. Although people with HIV/AIDS are less likely to get PCP today than in recent years, PCP is still a significant public health problem.

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PCP is a serious illness caused by infection with the fungus Pneumocystis jirovecii (pronounced NEW-mo-SIS-tis yee-row-VET-zee). It is one of the most common opportunistic infections in people with HIV/AIDS. Although people with HIV/AIDS are less likely to get PCP today than in recent years, PCP is still a significant public health problem.


The symptoms of PCP are fever, dry cough, shortness of breath, and fatigue. In people with weakened immune systems, PCP can be very serious, so it is important to see a doctor if you have these symptoms.

In HIV-infected patients, PCP usually presents sub-acutely, and symptoms include a low-grade fever. In HIV-uninfected patients, symptoms of PCP tend to develop more quickly and patients typically experience a high fever.

Risk & Prevention

Who gets pneumocystis pneumonia (PCP)?

PCP is extremely rare in healthy people. Most people who get PCP have weakened immune systems due to HIV/AIDS, cancer treatments, or organ transplants. Other groups of people who are at risk for PCP include:

  • HIV-exposed but uninfected children
  • People who are receiving immunosuppressive therapies, such as organ transplant patients
  • People with connective tissue diseases or chronic lung diseases
How can I prevent pneumocystis pneumonia (PCP)?

There is no vaccine to prevent PCP. Some groups of people who are at high risk of developing PCP may need to take a medication called TMP-SMX to prevent the illness from occurring. If your doctor thinks you are at risk for developing PCP, he or she might prescribe this medicine for you. TMP-SMX prophylaxis is currently recommended for:

  • All HIV-infected patients with CD4 < 350 cells / µL
  • Infants born to HIV-infected mothers
  • Children with a history of PCP
  • Stem cell transplant patients


Scientists are still learning about how people get PCP. Studies have shown that many people are exposed to the fungus as children, but they do not get sick because their immune systems are strong. Some healthy adults carry the fungus in their lungs and never develop symptoms of PCP. However, if a person's immune system stops working normally, the fungus can start causing symptoms.

Diagnosis and Testing

P. jirovecii cysts in a smear of bronchoalveolar lavage material.

P. jirovecii cysts in a smear of bronchoalveolar lavage material.

PCP is diagnosed by using a microscope to identify P. jirovecii organisms in a sample of lung fluid or tissue. The sample is usually induced sputum or bronchoalveolar lavage (BAL) material. It may be necessary to get the sample through transbronchial biopsy or open-lung biopsy – these diagnostic techniques are more sensitive and specific, but they are also more invasive.

Polymerase chain reaction (PCR) is also used to detect P. jirovecii DNA in clinical specimens. PCR can be particularly helpful in detecting silent P. jirovecii infections in HIV-infected patients.

Treatment and Outcomes

PCP requires treatment with prescription medicine that must be taken for three weeks. The best form of treatment for PCP is trimethoprim sulfamethoxazole (TMP-SMX), which is also known by the brand names Bactrim, Septra, and Cotrim. This medicine is given orally or through a vein.

TMP-SMX can cause negative side effects such as a rash and nausea, but the benefits of treating the PCP usually outweigh the risks of these side effects. Without treatment, PCP can be fatal.


Before the beginning of the HIV/AIDS epidemic in the 1980s, PCP was very uncommon. In fact, unusual clusters of PCP were one of the first signs that the HIV/AIDS epidemic was beginning. PCP soon became one of the main AIDS-defining illnesses in HIV-infected patients in the United States. Since then, the incidence of PCP in HIV/AIDS patients has declined in the U.S. due to the introduction of highly active antiretroviral therapy (HAART) and TMP-SMX prophylaxis. However, PCP is still a serious health concern for people with HIV/AIDS or other conditions that weaken the immune system.

In the U.S., the incidence of PCP is estimated to be 9% among hospitalized HIV/AIDS patients and 1% among solid organ transplant recipients. In immunocompromised patients, the mortality rate ranges from 5% to 40% in those who receive treatment. The mortality rate approaches 100% without therapy.

Additional Information

MMWR Articles:

Treating Opportunistic Infections among HIV-Infected Adults and Adolescents. MMWR Recommendations and Reports, December 17, 2004/53(RR15); 1-112.

Revised Guidelines for Prophylaxis against Pneumocystis carinii Pneumonia for Children Infected with or Perinatally Exposed to Human Immunodeficiency Virus. MMWR Recommendations and Reports, April 28, 1995/44(RR-4); 1-11.

Pneumocystis Pneumonia ---Los Angeles. MMWR Weekly, June 5, 1981/30(21); 1-3.


Harris, J. R., S. A. Balajee, et al. (2010). "Pneumocystis Jirovecii Pneumonia: Current Knowledge and Outstanding Public Health Issues." Curr Fungal Infect Rep 4(4): 229-237.

Kelley, C. F., W. Checkley, et al. (2009). "Trends in hospitalizations for AIDS-associated Pneumocystis jirovecii Pneumonia in the United States (1986 to 2005)." Chest 136(1): 190-197.

Medrano, F., M. Montes-Cano, et al. (2005). "Pneumocystis jirovecii in General Population." Emer Infect Dis 11(2): 245-250.

Sing, A., K. Trebesius et al. (2000). "Evaluation of Diagnostic Value and Epidemiological Implications of PCR for Pneumocystis carinii in Different Immunosuppressed and Immunocompetent Patient Groups." J Clin Microbiol 38(4): 1461-1467.

Wakefield, A. E., A. R. Lindley, et al. (2003). "Limited asymptomatic carriage of Pneumocystis jirovecii in human immunodeficiency virus-infected patients." J Infect Dis 187(6): 901-908.

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