HIV in Canada: A primer for service providers

Biology of Sexual Transmission of HIV

Key Points

  • For sexual HIV transmission to occur there are three necessary components: fluid, route, and activity.
  • Not all exposures to HIV lead to infection because of the body’s mucosal defences.
  • Several factors can affect the biological risk of HIV transmission.

For sexual HIV transmission to occur there are three necessary components: fluid, route, and activity. There needs to be a fluid from a person living with HIV that contains enough HIV to cause infection, a route within an HIV-negative person that HIV can use to enter that person’s body and an activity that brings the fluid and route together.

The fluids most commonly involved in the sexual transmission of HIV are: semen, pre-ejaculate or pre-cum, vaginal fluid and rectal fluid. The routes that are involved in the sexual transmission of HIV include the mucous membranes of the mouth and throat, vagina and cervix, urethra and foreskin, and the rectum. The activities that bring the fluid and route together during the sexual transmission of HIV include vaginal, anal, and oral sex.

After a sexual fluid containing HIV comes into contact with a mucous membrane, the virus must overcome the protective defences of the mucous membrane before it is able to spread throughout the body to cause a permanent infection. HIV cannot always overcome the mucosal defences and therefore an exposure does not always lead to infection.

The mucous membranes have several protective defences, including mucous (a slimy substance that covers the membrane, which can trap and kill germs), an epithelial cell layer (a tight layer of cells that can prevent germs from entering the body), and immune cells, which can fight and clear germs that enter the body. For HIV to cause infection after an exposure, it must pass the mucous and epithelial cell layer and replicate for one to three days in the mucous membrane tissue without being cleared by immune cells. If the virus can replicate for a sufficient period of time, it is then able to spread from the initial site of replication to other parts of the body and cause a permanent infection.

The amount of HIV in the fluid to which an individual is exposed may be the most important factor determining whether transmission occurs. This is known as the viral load. A higher viral load can increase the risk of HIV transmission.

Tearing and/or inflammation can also increase the risk for HIV transmission. Tearing can allow HIV to pass the epithelial cell layer and enter the body more easily. Inflammation can increase the concentration of immune cells in the mucous membranes of the oral, genital and rectal tissues. These immune cells can serve as target cells that help HIV pass the epithelial cell layer and allow for faster replication once the virus enters the mucous membrane tissue.

Tearing and/or inflammation can happen as a result of irritation, friction, douching/enemas, brushing teeth/flossing, spermicides, vaginal conditions (such as bacterial vaginosis or yeast infections), and sexually transmitted infections (STIs). While tearing and inflammation can increase risk, HIV is able to pass through the epithelial cell layer even if no tearing or inflammation is present.

It has been suggested that hormonal changes resulting from the use of hormonal contraceptives or from pregnancy and the hormonal changes associated with the menstrual cycle may increase the risk of HIV transmission, but the evidence is inconclusive. Several mechanisms have been suggested for how hormonal changes may increase the risk of HIV infection, including thinning of the cervical and vaginal lining and increasing the number of HIV target cells at the vagina and cervix.

This improved understanding of the biology of HIV transmission and factors that increase risk has led to the development of new biomedical HIV prevention interventions. These reduce the risk of HIV transmission by mitigating factors that increase risk, by maintaining the mucosal defences or by intervening in the steps that HIV must complete to cause infection.


From exposure to infection: The biology of HIV transmissionPrevention in Focus

HIV and the female genital tract – what does it mean for HIV prevention?Prevention in Focus


  1. Zuckerman RA, Whittington WLH, Celum CL et al. Higher concentration 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 Jul 1;190(1):156-61.
  2. Fox J, Fidler S. Sexual transmission of HIV-1. Antiviral Research. 2010 Jan;85(1):276-85.
  3. Haase AT. Early events in sexual transmission of HIV and SIV and opportunities for interventions. Annual Review of Medicine. 2011 Feb 18;62:127-39.
  4. Hladik F, Doncel GF. Preventing mucosal HIV transmission with topical microbicides: Challenges and opportunities. Antiviral Research. 2010 Dec;88 (Supplement 1):S3-9.
  5. Wilton J. From exposure to infection. The biology of HIV infection. Prevention in Focus. Fall 2011. Available from: