HIV is most commonly caught by having sex without a condom, or by sharing needles infected with the virus.
The main ways to catch HIV in the UK are from:
- having unprotected sex, including vaginal, oral and anal sex
- sharing sex toys with someone infected with HIV
- sharing needles among drug users
- 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
You can pass the virus to others whether or not you have any symptoms, and you are more infectious in the weeks following infection.
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:
- cuts and sores in the skin
- thin lining on or inside the anus and genitals
- thin lining of the mouth and eyes
You cannot catch HIV from an infected person from:
- kissing
- 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, so it cannot fight off infections.
HIV infects cells of the immune system, the body’s defence system, so it cannot 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 black Africa
- people who have had unprotected sex with a person who has lived in or travelled in black 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
Origins of the disease
HIV is thought to originate in Africa, where humans caught it from chimpanzees. The primates suffered from a very closely related virus called SIVcpz (simian immunodeficiency virus).
The SIVcpz virus spread to humans following contact with infected chimpanzee blood during the hunting of chimpanzees.
It is thought that for many years, the human form of HIV was limited to a remote part of Africa. However, new transports links enabled the virus to spread to other parts of Africa, before spreading across the world.
