HIV is usually transmitted sexually, although it can also be spread by sharing needles or from mother to child. For HIV transmission to occur as a result of sex between men, the following needs to be the case:

  • One of the men must have HIV (the section Is he likely to be HIV-positive? gives more detailed information about this) and have a viral load high enough to transmit the virus
  • The sex must involve body fluids (blood, cum or anal mucus) that contain sufficient quantities of HIV
  • These body fluids must get into the bloodstream of the negative man (the section How risky is...? has details about the kinds of sex that facilitate HIV transmission)

HIV transmission occurs when an HIV-negative man is exposed to HIV and the virus infects the cells in his blood. He then becomes HIV-positive. The HIV tests that are most frequently used in GUM clinics can detect HIV infection one month after HIV has been transmitted [1]. There is more information about this on our HIV Testing page.

What body fluids cause HIV to spread?
For HIV transmission to occur, HIV-infected body fluids have to pass into the bloodstream of an uninfected person. While HIV can be found in many different body fluids of a person with HIV, only some body fluids contain a sufficient quantity of the virus to enable HIV infection to occur.

These body fluids are:

  • blood
  • cum
  • pre-cum
  • discharge from STIs (such as gonorrhoea)
  • anal mucus - anal mucus is a naturally occurring fluid that lines the arse. Its main function in the body is to lubricate your shit as it passes. Research indicates that anal mucus is the body fluid with the highest concentration of HIV [2].

You cannot become infected with HIV through exposure to urine or saliva.

How does HIV enter the bloodstream?
There are two ways for HIV to get into the blood of an uninfected person:

  • directly into the bloodstream through damaged skin, injecting equipment or invasive surgical procedures 
  • or through mucous membranes

What are mucous membranes?
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Mucous membranes are thin tissues which protect openings in the human body. There are many mucous membranes in or on a person. They occur in places like the mouth, inside the eyelids, in the stomach and along the digestive tract. HIV can travel through the surface of a mucous membrane, enter the tiny blood vessels inside and attach itself to the mucous membrane. However, the mucous membranes that are most commonly involved in HIV transmission are in in the arse, the foreskin and head of the penis, the urethra (the tube you pee through), the mouth and the throat.

With the exception of the mouth and throat, none of the mucous membranes need to be damaged to provide an effective route for HIV transmission to occur. However, damage to the mucous membranes, like a sore or a cut, does make transmission more likely. Of the four mucous membranes that can allow HIV to enter the bloodstream, the mucous membrane in your bum provides the most effective route for HIV transmission. This is because the mucous membrane in your bum is designed to absorb liquids directly into the bloodstream. It is therefore extremely efficient at absorbing HIV. While a couple of studies have reported that circumcision is protective of gay men who are exclusive tops, studies indicate that this may not actually be the case and circumcised gay men can still catch HIV when the penis is exposed to HIV-infected fluids [3].

What makes HIV transmission more likely?

  • The situation that is most likely to result in HIV transmission is when a man with HIV who is not on treatment (whether or not he has been diagnosed) has sex an HIV-negative man without using a condom and/or he's not on PrEP and cums inside him. 
  • This situation is the most likely to mean HIV infection because a positive man (who is not HIV-undetectable) having sex a negative man without a condom or PrEP means that the negative man is exposed to HIV. 
  • The cum of an HIV-positive man, who is not on effective HIV treatment, is more likely to have a sufficient quantity of HIV in it to infect the negative man.   
  • The mucous membrane in the negative man's arse provides one of the most effective and efficient routes for HIV to enter the bloodstream [4].

More details about the risks of different sexual acts, both for HIV and for other STIs, can be found in the section How risky is...?.

LAST UPDATE: 6 September 2019




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1 Content 1 British Association for Sexual Health and HIV (BASHH). BASHH statement on HIV window period. British Association for Sexual Health and HIV, 15 March 2010.
2 Zuckerman RA, Whittington WLH, Celum CL, Collis TK, Lucchetti AJ, Sanchez JL, Hughes JP, Sanchez JL, Coombs RW. Higher concentrations of HIV RNA in rectal mucosa secretions than in blood and seminal plasma, among men who have sex with men, independent of antiretroviral therapy. Journal of Infectious Diseases, 2004;189:156-161.
Doerner R et al. Circumcision and HIV infection among men who have sex with men in Britain: the insertive sex role. Archives of Sexual Behavior, early online edition, DOI 10.1007/s10508-012-0061-1, 2013.
4 Royce RA, Sena A, Cates W Jr, Cohen MS. Sexual transmission of HIV. New England Journal of Medicine, 1997;336:1072-1078.