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HIV infection: Risk factors and prevention strategies

Myron S Cohen, MD
Section Editor
John G Bartlett, MD
Deputy Editor
Allyson Bloom, MD


Although the incidence of HIV has dropped considerably since the height of the epidemic, millions of new HIV infections occur globally each year. An understanding of the risk factors for HIV infection lies at the foundation of successful preventive strategies, which should combine both behavioral and biomedical interventions to reduce HIV infection risk.

This topic reviews these risk factors and prevention strategies. More detailed discussion on particular prevention strategies, including pre- and post-exposure prophylaxis for HIV, and prevention of mother-to-child transmission of HIV are found elsewhere. (See "Pre-exposure prophylaxis against HIV infection" and "Management of nonoccupational exposures to HIV and hepatitis B and C in adults" and "Management of healthcare personnel exposed to HIV" and "Antiretroviral and intrapartum management of pregnant HIV-infected women and their infants in resource-rich settings" and "Prevention of mother-to-child HIV transmission in resource-limited settings".)


HIV infection is acquired through sexual intercourse, exposure to infected blood, or perinatal transmission. The distribution of the modes of transmission of HIV infection varies in different countries. In the United States, male-to-male sexual contact and injection drug use (IDU) account for more than half of cases [1]. The estimated lifetime risk of an HIV diagnosis in the United States is 1 in 6 for men who have sex with men (MSM), 1 in 23 for women with IDU, and 1 in 36 for men with IDU, in contrast to 1 in 241 and 1 in 473 for heterosexual women and men, respectively [2,3].

In contrast, in resource-limited areas, penile-vaginal intercourse is responsible for 70 to 80 percent of HIV infections, perinatal transmission and IDU account for 5 to 10 percent each, and a smaller but growing proportion of cases are among MSM [4,5]. Since stigma remains a major problem hindering the study of MSM in resource-constrained countries, it seems clear that the incidence and prevalence of HIV in MSM in these settings needs far greater attention. (See "Global epidemiology of HIV infection", section on 'Modes of transmission driving the epidemic'.)


Risk of HIV infection varies by type of sexual or parenteral exposure (table 1). However, estimates of risk are mostly based on observational studies and are difficult to quantify since transmission risk also depends on other cofactors that greatly enhance (and occasionally reduce) the probability of infection [6-8]. As an example, risk factors for HIV transmission include high viral load in the source patient, and risk factors for HIV acquisition after exposure include sexually transmitted infections (STIs), and lack of circumcision, as well as certain host and genetic factors [9-11].


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