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Pneumonia carinii is an opportunistic organism. This means that it causes disease only under certain conditions, as when a person is immunocompromised. Under these circumstances, P. carinii can multiply and cause pneumonia. The mechanisms of the organism's growth within the alveoli are not fully understood. As the pneumocystis organism continues to replicate, it gradually fills the alveoli. As the pneumonia becomes more severe, fluid accumulates and tissue scarring occurs. These changes result in decreased respiratory function and lower levels of oxygen in the blood.
However, in individuals with weakened immune systems due to cancer, HIV/AIDS, solid organ and/or bone marrow transplantation, as well as individuals receiving chronic corticosteroids or other medications that affect the immune system, Pneumocystis carinii may lead to a lung infection.
Individuals with advanced AIDS are of particular interest, since PCP was a relatively rare infection prior to the AIDS epidemic. Before the use of preventive antibiotics for PCP, up to 70% of individuals in the U.S. with advanced AIDS would develop PCP.
PCP in the setting of AIDS usually develops slowly and is less severe -- several weeks of cough, fevers, and progressive shortness of breath, especially with exertion. Individuals with PCP who do not have AIDS usually get sick faster and are more acutely ill.
- Some patients are at greater risk of developing PCP. These high-risk groups include:
- premature infants
- patients with immunodeficiency diseases, including severe combined immunodeficiency disease (SCID) and acquired immunodeficiency syndrome (AIDS),
- patients receiving immunosuppressive drugs, especially cortisone-like drugs (corticosteroids)
- Patients with protein malnutrition.
- AIDS is currently the most common risk factor for PCP in the United States. PCP is, however, also found in countries with widespread hunger and poor hygiene.
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