HIV can be transmitted by sexual contact with an infected person; by needle sharing among infected intravenous drug users; through the receipt of infected blood, blood products, or tissue; and directly from an infected mother to her infant during pregnancy, delivery, or breastfeeding.

In the United States MSM (male to male sexual contact) remain the majority of HIV carriers, although prevalence among heterosexuals is on the rise. In 1987, 70% of adult and adolescent males with AIDS had a single risk factor of a history of high-risk sexual activity. Even though cumulatively more than half (54%) of reported cases of AIDS among male adults and adolescents were attributable to MSM in 2005, the percentage of affected MSM was 48%. Adult and adolescent males with a history of intravenous drug use as their only risk factor made up 14% of all cases in 1987. This proportion has remained relatively constant in the intervening years, and even though cumulatively 21% of reported AIDS cases in adult and adolescent males were attributable to intravenous drug use in 2005, it accounted for 13% of cases.

The proportion of adult and adolescent females with AIDS whose only risk factor was intravenous drug use has dropped from 50% in 1987 to 19% in 2005. Adult and adolescent females with a history of heterosexual contact as their only risk factor made up 38% of all female cases in 1997. By 2001 this proportion increased to 41%, and through 2005 it had increased to 45%. Researchers suggest that one reason for steadily increasing HIV infection and AIDS among heterosexuals is that an increased proportion report multiple sex partners, which is a risk factor for HIV infection.

In 2005 there were 10,857 adult and adolescent (both male and female) cases of AIDS with an undetermined risk. That is, there was no reported history of exposure to HIV through any of the routes listed in the exposure categories. These include people currently being investigated by local health departments, people whose exposure history was incomplete at the time of their death, those who refused to be interviewed or whose cases were not followed up, and those who were interviewed but for whom no follow-up occurred. When an exposure mode is identified during follow-up, patients are subsequently reclassified into the appropriate exposure category.

Arialdi M. Minin˜o, Melonie P. Heron, and Betty L. Smith, in ''Deaths: Preliminary Data for 2004'' (National Vital Statistics Reports, vol. 54, no. 19, June 28, 2006) note that in 2006 the average life expectancy for Americans was 77.9 years, an all-time high. This figure would have been higher were it not for heart diseases (the leading cause of death for all age categories), malignant neoplasms (cancers, which were the leading cause of death for those aged forty-five to sixty-four), and accidents (the leading cause of death for those aged fifteen to forty-four). AIDS is also among the top ten leading causes of death among those aged fifteen to forty-four. In HIV/AIDS Surveillance Report, 2005, the CDC states that from 2001 through 2005 AIDS had claimed the lives of 363,286 Americans aged fifteen to forty-four, representing two thirds (66%) of the 550,394 people who have died from AIDS.

Nearly 100% of AIDS patients die within seven years of the initial diagnosis of the late stage of HIV infection. Some deaths are not reported to the CDC or are reported as deaths from other causes. So the reported case-fatality rate (the number of deaths from a disease divided by the number of cases of that disease) is surely an underestimate. The case-fatality rate is frequently used as a measure of the severity of a disease and to estimate the probability of death among diagnosed cases.

The number of deaths due to AIDS peaked at 51,670 in 1995. Since then the number of deaths each year has been dropping. In 2005 the disease killed 17,011 Americans. Fewer people are dying from AIDS due to more effective treatment. As fewer people become infected with HIV, the death rate in subsequent years will drop proportionally. However, the statistics for children under the age of thirteen at the time of diagnosis remain grim: half die before their first birthday, whereas the other half do not live to adolescence.

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