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, the following needs to be the case:

  • Someone must have HIV 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 person

HIV transmission occurs when an HIV-negative person is exposed to HIV and the virus infects the cells in his blood. They 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]. 

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 faeces 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?
Read more 

What makes HIV transmission more likely?

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

Read more 
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.