Protists as opportunistic pathogens: Public health impact in the 1990s andbeyond

Proist organisms (protozoa and fungi) have become increasingly prominent as opportunistic pathogens among persons infected with human immunodeficiency virus (HIV) and among organ transplant recipients-two immunocompromised po pulations that have increased dramatically in the past two decades. Pneumoc ystis carinii pneumonia continues to be the most common serious opportunist ic infection (OI) among HIV-infected persons in the United States, occurrin g frequently among persons not previously receiving medical care. Toxoplasm osis, cryptococcosis, cryptosporidiosis, and isosporiasis occur frequently in HIV-infected persons in the developing world. Candidiasis and aspergillo sis are common OIs in organ transplant recipients. As these populations of immuno-suppressed patients continue to expand worldwide new OIs caused by p rotist pathogens are likely to emerge.