The organism Pneumocystis carinii causes pneumonia, especially in people with weakened immune systems.
By the time children turn 4 years old, 3 out of 4 will have acquired an organism called Pneumocystis carinii. People are believed to get it from the environment or from contact with others who are infected, but this is not known for sure.
Usually infection during early childhood causes no symptoms. When a child or adult has a weakened immune system, though, it may cause severe pneumonia. Pneumocystis carinii pneumonia, or PCP, is the most common serious infection by a normal inhabitant of the body found in children infected with the AIDS virus. Adults and children whose immune systems have been weakened by chemotherapy, AIDS, or other conditions can become severely ill with PCP. This is also true of malnourished and premature babies.
Pneumocystis carinii causes PCP. People with weakened immune systems are at risk for serious illness.