In the UK, most cases of HIV are caused by having sex without a condom with a person who has HIV.

A person with HIV can pass the virus to others whether or not they have any symptoms. People with HIV are more infectious in the weeks following infection.

HIV treatment significantly reduces the risk of someone with HIV passing it on.

Sexual contact

According to the Health Protection Agency, 95% of people diagnosed with HIV in the UK in 2011 acquired HIV through sexual contact.

The main routes of transmission are unprotected vaginal and anal sex. It is also possible to catch HIV through unprotected oral sex, but the risk is much lower.

The risk of HIV transmission through oral sex will be higher if the person giving oral sex has mouth ulcers, sores or bleeding gums and/or if the person receiving oral sex has been recently infected with HIV (and has a lot of the virus in their body) or another sexually transmitted infection.

The type of sex also makes a difference to the level of risk:

  • performing oral sex on a man with HIV carries some risk, particularly if he ejaculates (comes) in your mouth
  • it is possible to catch HIV by performing oral sex on a woman with HIV, particularly if she is having a period, although this is considered to be extremely low risk
  • receiving oral sex from someone who has HIV is also extremely low risk as HIV is not transmitted through saliva

Other forms of transmission

Other ways of getting HIV include:

  • sharing needles, syringes and other injecting equipment 
  • from mother to baby before or during birth or by breastfeeding
  • sharing sex toys with someone infected with HIV
  • healthcare workers accidentally pricking themselves with an infected needle (this risk is extremely low)
  • blood transfusion (now very rare in the UK, but still a problem in developing countries)

How HIV spreads

HIV is not passed on easily from one person to another. The virus does not spread through the air like cold and flu viruses.

HIV lives in the blood and in some body fluids. To get HIV, one of these fluids from someone with HIV has to get into your blood.

The body fluids that contain enough HIV to infect someone are:

  • semen
  • vaginal fluids, including menstrual blood
  • breast milk
  • blood
  • lining inside the anus

Other body fluids, like saliva, sweat or urine, do not contain enough of the virus to infect another person.

The main ways the virus enters the bloodstream are: 

  • by injecting into the bloodstream (with a contaminated needle or injecting equipment)
  • through the thin lining on or inside the anus and genitals
  • through the thin lining of the mouth and eyes
  • via cuts and sores in the skin

HIV is not passed on through:

  • kissing
  • spitting
  • being bitten
  • contact with unbroken, healthy skin
  • being sneezed on
  • sharing baths, towels or cutlery
  • using the same toilets and swimming pools
  • mouth-to-mouth resuscitation
  • contact with animals or insects such as mosquitoes

How HIV infects the body

HIV infects cells of the immune system, the body’s defence system, making it unable to fight off infections.

The virus enters the immune system’s CD4 cells, which protect the body against various bacteria, viruses and other germs.

It uses the CD4 cells to make thousands of copies of itself. These copies then leave the CD4 cells, killing them in the process.

This process continues until eventually the number of CD4 cells, also called your CD4 count, drops so low that your immune system stops working.

This can take about 10 years, during which time you will feel and appear well. Find out more about the symptoms of HIV.

Risk groups

People who are at high risk of catching HIV include:

  • men who have had unprotected sex with men
  • women who have had sex without a condom with men who have sex with men
  • people who have lived or travelled extensively in Africa 
  • people who have had sex without a condom with a person who has lived or travelled in Africa
  • people who inject drugs
  • people who have had sex without a condom with somebody who has injected drugs
  • people who have caught another sexually transmitted infection
  • people who have received a blood transfusion while in Africa, eastern Europe, the countries of the former Soviet Union, Asia or central and southern America