AIDS was first recognized in the United States in the summer of 1981, when the U.S. Centers for Disease Control and Prevention (CDC) reported the unexplained occurrence of Pneumocystis carinii pneumonia in five previously healthy homosexual men in Los Angeles and of Kaposi's sarcoma (KS) in 26 previously healthy homosexual men in New York and Los Angeles. Within months, the disease became recognized in male and female injection drug users (IDUs) and soon thereafter in recipients of blood transfusions and in hemophiliacs. As the epidemiologic pattern of the disease unfolded, it became clear that a microbe transmissible by sexual (homosexual and heterosexual) contact and blood or blood products was the most likely etiologic agent of the epidemic.
In 1983, HIV was isolated from a patient with lymphadenopathy, and by 1984 it was demonstrated clearly to be the causative agent of AIDS. In 1985, a sensitive enzyme-linked immunosorbent assay (ELISA) was developed, which led to an appreciation of the scope of HIV infection among cohorts of individuals in the United States who admitted to practicing high-risk behavior as well as among selected populations that had been screened, such as blood donors, military recruits and active-duty military personnel, Job Corps applicants, and patients in selected sentinel hospitals. In addition, seroprevalance studies revealed the enormity of the global pandemic, particularly in developing countries.
ETIOLOGIC AGENT
The etiologic agent of AIDS is HIV, which belongs to the family of human retroviruses (Retroviridae) and the subfamily of lentiviruses. Nononcogenic lentiviruses cause disease in other animal species, including sheep, horses, goats, cattle, cats, and monkeys. The four recognized human retroviruses belong to two distinct groups: the human T lymphotropic viruses (HTLV) I and HTLV-II, which are transforming retroviruses; and the human immunodeficiency viruses, HIV-1 and HIV-2, which are cytopathic viruses. The most common cause of HIV disease throughout the world, and certainly in the United States, is HIV-1. HIV-1 comprises several subtypes with different geographic distributions. HIV-2 was first identified in 1986 in West African patients and was originally confined to West Africa. However, a number of cases that can be traced to West Africa or to sexual contacts with West Africans have been identified throughout the world. HIV-2 is more closely related phylogenetically to the simian immunodeficiency virus (SIV) found in sooty mangabeys than it is to HIV-1. In 1999, it was demonstrated that HIV-1 infection in humans was zoonotic and had originated from the Pan troglodytes troglodytes species of chimpanzees in whom the virus had co-evolved over centuries.
