HIV lives in the following areas of the body:

  • Blood
  • Semen and vaginal fluid
  • Breast milk

To be passed on from one person to another, the virus needs:

  • to be present in one of the bodily fluids above in sufficient quantity to be transmitted
  • a route into a person’s bloodstream.

In the past lots of people got HIV through blood transfusions, but this was before it was known how the virus was passed on. In most places in the world now, blood is treated before being used in transfusion as there are lots of things that can be passed on in this way, not only HIV.

If a person who is living with HIV has sex (vaginal or anal) without a condom, the HIV in the semen or vaginal fluid can enter their partner’s body and get into their bloodstream. When a person living with HIV is on effective treatment, the treatment lowers the level of HIV in their blood. If a person’s viral load is sufficiently lowered (known as ‘undetectable’), HIV cannot be passed on sexually. This is known as ‘U=U’ which stands for ‘undetectable = untransmittable’ (learn more). Not everyone taking HIV treatment will have an undetectable viral load.

If a person with HIV injects themselves with a needle, then another person injects themselves with the same needle immediately after, HIV can be passed on. People who are intravenous drug users and share needles (without sterilising them after each person’s use) have a very high risk of either getting infected by HIV or passing it on.

Vertical transmission
HIV can be passed onto a baby at three stages in their development if the mother is living with HIV:

  • Whilst they are growing in the womb
  • During childbirth
  • Through breastfeeding

There are now steps that a woman living with HIV can take to help stop her baby being born with HIV. These work so well that in the UK now, there is less than a 1 in 100 chance of HIV being passed from an HIV positive mother to her baby.

The interventions to prevent vertical transmission of HIV (sometimes also called perinatal transmission) are as follows:

  • Knowing the mother has HIV, i.e. testing and diagnosis.
  • The mother taking HIV medication during her pregnancy so that her viral load (the amount of HIV detected in her blood) becomes ‘undetectable’. This greatly reduces the risk of HIV transmission.
  • She either has a controlled vaginal delivery or an elected caesarean section to reduce the potential for trauma at the time of birth. Trauma during birth can increase the risk of HIV transmission as a lot of bodily fluid exchanges can occur.
  • The baby is given HIV medication for a few weeks after birth.

Breast milk
HIV can be found in breast milk, so if a mother is living with HIV, in the UK it is recommended she shouldn’t breastfeed her baby unless it has been advised as safe to do so by a medical professional. For a mother with an undetectable HIV viral load she may choose to breastfeed with support from medical teams. More information can be found in the BHIVA guideline on the management of HIV in pregnancy and postpartum.

HIV cannot be transmitted through:

  • Unbroken healthy skin – HIV cannot pass through unbroken skin and enter the bloodstream
  • Droplet or airborne transmission – e.g. sneezing, coughing, breathing
  • Saliva – e.g. sharing cutlery, plates or cups
  • Mosquitos – although mosquitos suck blood, they do not regurgitate blood into the next person they bite
  • Social or household contact with people who are living with HIV, as HIV is not transmitted by touch or through the air
  • An unbroken barrier such as a latex condom.