Note from Dr. Merle A. Sande—As the relentless human immunodeficiency virus (HIV)induced destruction of T4lymphocytes progresses, the human host eventually becomes defenseless against his or her own unique microbic flora acquired during a lifetime of exposure. These organisms are controlled but not totally eliminated by the host's once functionally intact cellular immune system. Reactivation of these various parasites probably leads initially to local proliferation, then often to widespread dissemination and subsequent complicating infections that make up most clinical conditions by which AIDS is defined: Pneumocystis carinii pneumonia, tuberculosis and other mycobacterial infections, histoplasmosis, coccidioidomycosis, toxoplasmosis, and cytomegalovirus and other herpesvirus infections. Thus, the spectrum of infections with which members of a given AIDS population present reflects that population's lifetime exposure history and would beexpected to vary in different parts of the world. Such is the case, as discussed by Dr. Joan K. Kreiss, University of Washington School of Medicine, Seattle, who has spent considerable time studying HIV diseases in Africa, and Dr. Kenneth G. Castro of the Division of HIV/AIDS at the Centers for Disease Control's Center for Infectious Diseases in Atlanta, who has helped track the epidemic in developing countries.