Homosexual or bisexual men constitute the largest group, accounting for about 50% of the reported cases. This includes about 5% who also are intravenous drug abusers.
Heterosexual contacts of members of other high-risk groups constituted about 20% of infections from 2001 to 2004. In Africa and Asia, this is by far the largest group of patients with new infections, most of which occur in women who are infected by male partners.
Intravenous drug abusers with no previous history of homosexuality represented about 20% of infected individuals and 9% of new cases in 2009.
Surviving hemophiliacs, especially those who received large amounts of factor VIII or factor IX concentrates before 1985, make up about 0.5% of all cases.
Other recipients of HIV-infected whole blood or components (e.g., platelets, plasma) account for about 1% of patients.
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HIV infection of the newborn. Close to 2% of all AIDS cases occur in this pediatric population. The vast majority acquires the infection by transmission of the virus from mother to child (discussed later).
In approximately 5% of cases, the risk factors cannot be determined.
It should be apparent from the preceding discussion that transmission of HIV occurs under conditions that facilitate exchange of blood or body fluids containing the virus or virus-infected cells. The three major routes of transmission are sexual contact, parenteral inoculation, and passage of the virus from infected mothers to their newborns.
Sexual transmission is the dominant mode of infection worldwide, accounting for more than 75% of all cases of HIV transmission. Because most infected individuals in the United States are men who have sex with men, sexual transmission has mainly occurred among homosexual men. The virus is carried in the semen, and it enters the recipient’s body through abrasions in rectal or oral mucosa or by direct contact with mucosal lining cells. Viral spread occurs in two ways: (1) direct inoculation into the blood vessels breached by trauma and (2) infection of DCs or CD4+ cells within the mucosa. Sexual transmission of HIV is enhanced by coexisting sexually transmitted diseases, especially those associated with genital ulceration.
Parenteral transmission of HIV has occurred in intravenous drug abusers, hemophiliacs who received contaminated factor VIII and factor IX concentrates, and other recipients of contaminated blood products. In the United States at present, parenteral transmission is common only among intravenous drug users. Transmission occurs by sharing of needles, syringes, and other paraphernalia contaminated with HIV-containing blood. Transmission of HIV by transfusion of blood or blood products, such as lyophilized factor VIII and factor IX concentrates, has been virtually eliminated by public health measures, including screening of donated blood and plasma for antibody to HIV, stringent purity criteria for factor VIII and factor IX preparations, and screening of donors on the basis of history. An extremely small risk for acquiring AIDS through transfusion of seronegative blood persists, because a recently infected individual may be antibody-negative. Currently, this risk is estimated to be 1 in more than 2 million units of blood transfused.