Causes of HIV

In the UK, most cases of HIV are caused by having unprotected sex 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.

Sexual contact

According to the Health Protection Agency, 95% of people diagnosed with HIV in the UK in 2010 acquired HIV through sexual contact.

The main routes of transmission are unprotected vaginal and anal sex. although it is also possible to catch HIV through unprotected oral sex with a person who has HIV, but the risk is much lower than that for vaginal and anal sex.

If a person performing oral sex has HIV, transmission of the virus can occur if blood from their mouth passes into the body of the other person. If the person receiving oral sex has HIV, transmission can occur if their semen, vaginal fluid or blood gets into the mouth of the other person.

Other forms of transmission

Other ways of getting HIV include:

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

• thin lining on or inside the anus and vagina
• thin lining of the mouth and eyes

You cannot catch HIV from an infected person from:

• spitting
• 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

If someone with HIV has bleeding in their mouth, such as from open ulcers, sores or cuts, there is a very low chance that this could lead to transmission during open-mouthed kissing. HIV spread through biting is very rare, with only a couple of documented cases.

Read more frequently asked questions about HIV transmission on the AVERT website.

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 unprotected sex with men who have sex with men
• people who have lived in or travelled extensively in Africa
• people who have had unprotected sex with a person who has lived in or travelled in Africa
• people who inject illegal drugs
• people who have had unprotected sex with somebody who has injected illegal 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