HIV transmission


For HIV transmission to occur three things are needed: a fluid, a route and an activity or event.

First, there needs to be a fluid that contains enough HIV to cause infection. The amount of HIV in a fluid is called the viral load. The lower the viral load, the lower the chance of passing HIV. Only five bodily fluids can contain enough HIV to transmit the virus: blood, semen (including pre-cum), rectal fluid, vaginal fluid and breast milk.

Second, HIV can only be passed when virus in one of the five fluids gets into the body of an HIV-negative person through a mucus membrane or a break in the skin. This is the route. Mucus membranes are the wet linings of the body, such as the opening of the penis, the foreskin, the vagina or the rectum. A break in the skin can happen when someone shares needles used to inject drugs or someone has a needlestick injury.

Third, there needs to be an activity or event that brings the fluid and route together for HIV transmission to occur. The activities/events that can transmit HIV are:

  • sexual activity (such as anal or vaginal sex) without the use of a highly effective HIV prevention strategy
  • sharing/reusing needles or other equipment used to inject drugs or used for tattooing or piercing, and accidental needlestick injury
  • pregnancy, childbirth and infant feeding (for transmission to a fetus or infant)

HIV is not transmitted by saliva, tears, sweat, urine or feces. HIV does not survive well outside the human body. It cannot be transmitted through casual contact with a person who has HIV or through objects such as toilet seats, doorknobs or dishes used by a person who has HIV.

It is difficult to precisely estimate a person’s chance of getting HIV because there are many factors that can increase or decrease risk. A person’s risk of HIV transmission depends on the activities that they are participating in, the frequency that they participate in these activities, certain biological factors that can increase or decrease risk, whether an HIV prevention strategy is used, and various social and structural factors that can influence risk.

To learn more about HIV transmission, check out:

HIV transmission – This fact sheet discusses the conditions that are necessary for HIV transmission to be possible and explains how transmission can happen through sex, sharing drug use equipment and during pregnancy, labour and breastfeeding.

Assessing and addressing HIV risk: What service providers need to know – This article in CATIE’s publication Prevention in Focus explores the risk of HIV transmission under different circumstances and considerations when talking to clients about HIV transmission risk.

HIV Basics – This video for clients and service providers explains how HIV works in the body, and includes key messages about testing, treatment and prevention.

Preventing the sexual transmission of HIV – This self-directed course provides in-depth information about the highly effective ways to prevent HIV.