Scientific research shows that the human immunodeficiency virus (HIV) is transmitted by contaminated body fluids such as blood, semen (including pre-ejaculatory fluid), vaginal fluids and breast milk. Unprotected sex is one of the leading modes of transmission.
A heterosexual person infected with HIV will transmit the virus to their partner once in every 900 times the couple has unprotected sex, according to a new study conducted in Africa.
However, the exact number of sexual acts that are needed to transmit the virus can vary tremendously depending on the amount of the virus in the infected person’s blood, said study researcher James Hughes, of the University of Washington in Seattle.
In fact, the amount of virus in the blood is the single most important factor in determining whether HIV is passed between sexual partners, the study found. For every tenfold increase in the concentration, there is about a threefold increase in the risk of transmission during a single sexual act.
People with very high blood concentrations of the virus (such as those who very recently acquired the infection) may need to have sex only 10 times to transmit the virus, Hughes said. “The average can be a little deceptive,” Hughes said.
The new findings reinforce the idea that the best methods for reducing HIV transmission are those that decrease the concentration of the virus in the blood, as can be done with antiretroviral drugs, Hughes said. A study published last year found the drugs could reduce the transmission of HIV between partners by 96 percent.
These are acts that present no risk of HIV transmission because there is no exchange of body fluids and no transmission of HIV has been documented:
French kissing when no blood is present (saliva is not a body fluid that can transmit HIV)
masturbating (oneself or someone else)
using a sex toy and not sharing it.
The new study also confirmed condoms are highly effective in preventing HIV infection, reducing the risk of transmission by 78 percent. Male circumcision reduced the risk of HIV transmission by 47 percent.
These acts present a risk of HIV transmission because they lead to an exchange of body fluids that can transmit HIV. Infection cases have been documented for acts like:
fellatio given without a condom (when semen or pre-ejaculatory fluid is present)
cunnilingus given without protection (whether during menstruation or not)
vaginal penetration with a condom (risk exists for both partners)
anal penetration with a condom (risk exists for both partners).
French kissing when blood is present.
Earlier studies attempted to estimate the rate of HIV transmission, but were typically quite small, and did not measure the concentration of the virus in the blood throughout the entire study period.
Sexual intercourse with an HIV-infected person
HIV infection is sexually transmitted primarily through unprotected vaginal or anal intercourse (i.e. sexual intercourse without a condom), as well as through oral sex under certain conditions.
How does HIV get into the body during sexual contact?
To gain entry into the body of an uninfected person, the HI virus needs to bind or “latch onto” target cells with specific receptors (namely CD4 receptors) on its surface. Cells with these special receptors are plentiful in the lining of the genital track and that of the anus.
During unprotected sex with an HIV+ person, the virus (in the sexual fluids of the infected person) binds with the CD4 receptors in the lining of the genital tracks (e.g. vagina) or anal track of the uninfected partner.
Tears (often microscopically small) in the membrane linings of the genital tracks - especially in the anal-rectal area - also make it easy for the virus to enter the sex partner’s bloodstream. Because the membrane linings of body cavities - especially in the anal-rectal area, and, to a lesser extent, in the vagina - are very delicate, they can be torn as a result of friction generated during sexual intercourse. (Rough sex, dry sex and forced sex or rape often lead to friction, tears and bleeding.)
Sexually transmitted infections (STIs), such as syphilis, gonorrhoea or herpes, make it very easy for HIV to get into the body. An untreated STI in either partner increases the risk of HIV transmission during unprotected intercourse ten-fold.
People with genital herpes or genital ulcers or sores are especially susceptible to HIV infection because these conditions create openings in the linings of the genital tracks through which HIV can move. The discharges produced by many STIs contain a very high concentration of HIV if that person is also HIV positive.
The new study included 3,297 couples from sub-Saharan Africa that were “HIV-discordant,” meaning one partner had HIV while the other did not. The HIV-infected partners in the study were tested periodically over the two-year study for the amount of HIV in their blood. Infected partners were also interviewed every month and asked how many times they had sex, and whether they used protection.
Are there risks of getting HIV if I drink or use drugs?
Yes! Alcohol or drugs won’t infect you with HIV, but taking risks while you’re drunk or high might.
When you drink or use drugs, you may not make the same decisions as you would when not under the influence. For example, you may not bother to use protection during sex. Having unprotected sex is a common way of getting HIV.
You may also take the risk of sharing a needle to inject drugs. About one quarter of all new HIV infections occur among people who inject drugs. This is due to the invisible amount of blood that is in the needle or syringe or other injection equipment such as cookers, water, cotton filters, straws and pipes. If you use the same needle as someone who is infected with HIV, you shoot her/his infected blood into your bloodstream.
Remember that poppers (nitrite inhalants) and erectile dysfunction medications increase blood flow by dilating the blood vessels in the pelvic area, making the skin of the vagina and anus thinner and weaker, and therefore easier to tear. Tiny tears in these tissues make it easier for viruses to enter your bloodstream, increasing your risk for HIV infection.
Revision date: June 18, 2011
Last revised: by Sebastian Scheller, MD, ScD