Transmission through sexual acts

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.

How many sexual contacts with an HIV-positive person are necessary before one becomes infected oneself?
It is impossible to say how many sexual contacts with an infected partner/s are necessary before an HIV-negative person becomes HIV-positive.

HIV transmission from one person to another through sexual contact can depend on any of the following factors:

  • Unprotected (without a condom) sex with multiple sex partners.  
  • The presence of other STIs (sexually transmitted infections), especially in the case of genital ulcers and inflammatory STIs.  
  • Trauma (or bleeding) during sex, as well as menstruation.  
  • The viral concentration (or level of viral “load”) in the blood of the infected partner.  
  • The level of viral load in the semen or vaginal fluids of the infected partner (Viral load in semen peaks three weeks after infection. HIV is 20 times more transmissible per sex act, at this stage.)  
  • The phase of infection. HIV is much more contagious in the acute infection phase (first eight to 12 weeks after infection) as well as in the final phases when advanced disease (or Aids) has set in. The reason why HIV is easier transmitted during these phases, is because the viral load is especially high in the acute infection phase, and again in the final phase of Aids.  
  • When the immune system is suppressed (this is also then an indication of a high viral load and a low CD4 count.  
  • Other diseases such as malaria. The HIV viral load is very high in people who also have malaria.  
  • HIV-1 subtype C is more infectious than other subtypes, and subtype C is unfortunately the viral subtype that occurs in South Africa.

Is it true that circumcision can protect males against HIV infection?
Several studies reported at the Aids conference in Barcelona (2002) indicated that circumcision can indeed reduce the risk of HIV infection for the circumcised male, but:

  • It seems that circumcision only provides a measure of protection if the circumcision was done before the boy reached puberty (cancel those appointments men!)  
  • The direction of effectivity is not clear yet (in other words, will male circumcision also lower the risk of male to female transmission - and protect the sex partner?)  
  • If the circumcision procedure is done with unsterilised, dirty blades (or other instruments) - as often happens in Africa - HIV-infected blood could pass from one boy (or male) to another.  
  • Although circumcision may lower the risk of HIV infection in circumcised men, circumcision should definitely not be promoted as the only way of preventing HIV infection! Safer sex practices such as the use of condoms should always be practised, disregarding circumcision or not!

If I am HIV+, why should I still use a condom to protect myself?
HIV+ individuals still need to protect themselves against re-infections with HIV, for the following reasons:

  1. Each new infection can cause an increase in the viral load in the blood and  
  2. The person can become infected with a new strain of the virus. It is therefore important for HIV+ people to protect themselves from re-infection by always using condoms.

Can lesbians get Aids?
Yes, lesbian women can also get Aids.

Lesbians who have sex with women as well as men, can contract HIV infection if they have sex with infected men.

Although the risk of HIV-infection is small for women who have sex exclusively with other women, lesbian women can contract HIV infection by sharing contaminated sex toys (e.g. vibrators or dildo’s) for vaginal or anal penetration. Because sex toys can cause bleeding or irritation of the vaginal or anal lining, it is easier for the HI virus to enter the body if people use them or share them. When sex toys are used, they should be thoroughly cleaned and preferably not shared, but if they are shared, the sex toy should be covered with a condom (a new condom for each partner!).

It should also be kept in mind that menstruation blood of an HIV+ woman is contagious. A plastic/latex cover such as “glad wrap”, a dental dam, or a condom that is cut open, should be placed over the vagina for oral sex (also for heterosexual oral sex with a woman).

Myron S. Cohen

  Dr. Myron S. Cohen, University of North Carolina School of Medicine, CB #7030, 130 Mason Farm Rd., 4123 Bioinformatics Bldg., Chapel Hill, NC 27599-7030 (.(JavaScript must be enabled to view this email address)).
  Presented in part: Opportunities for Improving HIV Diagnosis, Prevention & Access to Care in the U.S., Washington, D.C., 29–30 November 2006.

The average risk of HIV infection per sexual act estimated in the study is consistent with what has been found by previous research, but there are many situations in which that number may not apply,” said Dr. Myron Cohen, a professor of medicine, microbiology, immunology and public health at the University of North Carolina at Chapel Hill, who was not involved in the work.

That’s because the participants included in the study are couples that have remained together and discordant over a long period of time. This indicates the couples might have some biological protection against transmission, Cohen said.

“The true estimation might be higher if you were studying different kinds of people,” Cohen said.

While most of the findings are likely generalizable to other countries, the number of sexual acts needed to transmit the virus is likely specific to the African population studied, Hughes said. Previous studies in the United States have found a lower transmission rate.

In addition, the findings only apply to heterosexual couples, and not men who have sex with men, a group that is likely to have a much higher transmission rate, Hughes said.

Pass it on: The best way to reduce the risk of HIV transmission is to lower the amount of the virus in the blood.


  Cameron DW,
  Simonsen JN,
  D’Costa LJ,
  et al

. Female to male transmission of human immunodeficiency virus type 1: risk factors for seroconversion in men.

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Provided by ArmMed Media
Revision date: July 3, 2011
Last revised: by Andrew G. Epstein, M.D.