Opportunistic infections caused by protozoan parasites.

Parasitic protozoa have played a major role in the discovery of the HIV epidemic. It was the occurrence of clustered cases of Pneumocystis pneumonia which caught the attention of the epidemiologists indicating that something unusual was going on. Very recently remarkable progress was made in understanding the epidemiology. Knowledge is evolving that Pneumocystis is host-specific to an extent which seems to justify species rank for the cause of death in AIDS patients for many years. Toxoplasma gondii infections were also recognized very early in AIDS patients. Cerebral toxoplasmosis has now replaced Pneumocystis pneumonia as a fatal infection in many cases. AIDS patients with CD4 cells below 200/microl have a high risk to develop this complication. Cryptosporidium parvum was recognized as the cause of diarrhea in AIDS patients and also in immunocompetent humans where it only leads to a diarrheal episode lasting a few days. In AIDS patients the persisting diarrhea leading to loss of fluid and electrolytes can be life-threatening. Another group of parasites previously unknown to infect humans are the microsporidia, which are wide-spread parasites in nature. Increasing numbers of such infections are observed but many questions regarding pathogenesis and epidemiology are still unclear.