Key Points

• Human immunodeficiency virus is transmitted through mucous membranes during anal, vaginal, and oral intercourse.

• Transmission of HIV also occurs through sharing injection equipment and needles. Vertical transmission occurs from mother to child.

• Blood, semen, vaginal fluid, breast milk, cerebrospinal fluid, amniotic fluid, and serosanguineous fluid can transmit HIV; saliva, tears, and sweat do not.

• The virus cannot survive outside the host, and 90% to 99% of infective HIV on dry surfaces is eliminated within hours. Insect bites and casual contact carry no risk.

As epidemic and scientific approaches to HIV infection evolved, the focus shifted from "risk groups" to "risky behaviors." This distinction is important, because risk groups can give a false sense of security by implying that certain groups of individuals are less vulnerable to infection. Risky behaviors is a more useful term, falling along a spectrum of "no risk" (e.g., complete sexual abstinence), "very low risk" (e.g., 100% use of latex condoms), to "very high risk" (e.g., unprotected receptive anal intercourse with ejaculation), with other behaviors in midspectrum. The physician serves as a source of accurate information to be provided in simple, nonjudgmental terms because ultimately the patient will decide the acceptable degree of risk.

The three main modes of transmission of HIV are as follows: 1. Through the mucous membranes during anal, vaginal, or oral sexual intercourse with an HIV-infected person. Average risk of transmission in heterosexual exposure is 1 in 1000 and increases with commercial sex workers to about 5 to 10 in 100. Co-infection with other sexually transmitted diseases (STDs), rough sex, and higher viral load increase risk, whereas condom use and male circumcision reduce risk. Uncircumcised men run risk similar to a woman, and circumcised men may transmit the virus to female partners four times as efficiently as uncircumcised men. Postmenopausal women are more susceptible because of thinning of the vaginal mucosa. Risk of infection per act has been suggested from cohort studies, so physicians should avoid discussing risks in numeric terms.

2. Through the veins while sharing needles or injection paraphernalia with an HIV-infected person.

3. Vertically from an HIV-infected mother to an infant during pregnancy, delivery, or through breastfeeding. High viral load, prolonged time after rupture of membranes, and chorioamnionitis increase risk of transmission, and peripartum prophylactic antiretro-viral therapy decreases risk. Overall risk of transmission by breastfeeding is about 15% to 25% in 18- to 24-month-old infants.

Occupational exposure occurs by needle stick injuries (risk, 3:1000), infected blood or fluid splashing into the mouth or nose, or exposure to infected blood through a cut or an open wound. Mucous membrane exposure carries a risk of infection of about 9 in 10,000. Transmission of HIV through infected blood is extremely rare after routine screening of the blood supply was initiated in 1985. With risk of transmission as low as 2 per million, 16 annual infections are accounted for by infectious donations. Neither insect bites nor casual contact carry any risk.

Human immunodeficiency virus can be transmitted by blood, semen, vaginal fluid, breast milk, and serosan-guineous body fluids. Contact with cerebrospinal fluid (CSF), amniotic fluid, and synovial fluid can be a risk factor for HIV transmission. Importantly, although HIV can be present in small quantities in saliva, sweat and tears, contact with these fluids does not transmit HIV. The virus cannot survive outside the host, and the amount of infective virus dried on surfaces is reduced by 90%-99% in a few hours.

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