How HIV is passed on...

Vaginal sex

HIV is found in the sexual fluids of an infected person. For a man, this means the pre-come and semen fluids that come out of the penis before and during sex. For a woman, it means HIV is in the vaginal fluids which are produced by the vagina to keep it clean and to help make intercourse easier.

If a man with HIV has vaginal intercourse without a condom then HIV can pass into the woman's body through the lining of the vagina, cervix and womb. The risk of HIV transmission is increased if the woman has a cut or sore inside or around her vagina; this will make it easier for the virus to enter her bloodstream. Such a cut or sore might not always be visible, and could be so small that the woman wouldn't know about it.

If a woman with HIV has sexual intercourse without a condom, HIV could get into the man's body through a sore patch on his penis or by getting into his urethra (the tube that runs down the penis) or the inside of his foreskin (if he has one).

Any contact with blood during sex increases the chance of infection. For example, there may be blood in the vagina if intercourse occurs during a woman's period. Some sexually transmitted infections – such as herpes and gonorrhoea – can also raise the risk of HIV transmission.

Anal sex

Receptive anal intercourse (i.e. being the “bottom”) carries a higher risk of HIV transmission than receptive vaginal intercourse. The lining of the anus is more delicate than the lining of the vagina, so is more likely to be damaged during sex. Any contact with blood during sex increases the risk of infection.

If a man takes the insertive (“top”) position in anal sex with a man or woman who has HIV, then he too risks becoming infected.

Oral sex

Oral sex with an infected partner does carry a small risk of HIV infection. If a person gives oral sex (licking or sucking the penis) to a man with HIV, then infected fluid could get into their mouth. If the person has bleeding gums or tiny sores or ulcers somewhere in their mouth, there is a risk of HIV entering their bloodstream.

The same is true if infected sexual fluids from a woman get into the mouth of her partner.

There is also a small risk if a person with HIV gives oral sex when they have bleeding gums or a bleeding wound in their mouth. Saliva does not pose a risk.

HIV infection through oral sex alone seems to be very rare, and there are things you can do to protect yourself. For more information go to our HIV and oral sex page.

Injecting drugs

Injecting drug users are a high-risk group for exposure to HIV. Sharing injecting equipment is a very efficient way to transmit blood-borne viruses such as HIV and Hepatitis C. Sharing needles and “works” (syringes, spoons, filters and blood-contaminated water) is thought to be three times more likely to transmit HIV than sexual intercourse. Disinfecting equipment between each use can reduce the chance of transmission, but does not eliminate it entirely.

For more information, go to our injecting drugs, drug users and HIV page.

Mother to child transmission

An infected pregnant woman can pass HIV on to her unborn baby during pregnancy, labour and delivery. HIV can also be transmitted through breastfeeding.

If a woman knows she is infected with HIV, there are drugs she can take to greatly reduce the chances of her child becoming infected.

For more information, go to our pages about pregnancy and mother-to-child transmission of HIV.

Blood transfusions

Some people have been infected through a transfusion of infected blood. These days, in developed countries all the blood used for transfusions is tested for HIV. In those countries where the blood is tested, HIV infection through blood transfusions is now extremely rare. In some developing countries, testing systems are not so efficient and transmission through blood transfusions continues to occur.

Blood products

Blood products, such as those used by people with haemophilia, are now heat-treated to make them safe.

Infection in the health-care settings

Hospitals and clinics should take precautions to prevent the spread of blood-borne infections. These measures include using sterile surgical instruments, wearing gloves, and safely disposing of medical waste. In developed countries, HIV transmission in health-care settings is extremely rare. However, cases continue to occur in less-resourced areas where safety procedures are not so well implemented.

Health-care workers have on rare occasions become infected with HIV by being stuck with needles containing HIV-infected blood. A few have also become infected by HIV-infected blood getting into the bloodstream through an open cut, or splashing onto a mucous membrane (e.g. the eyes or the inside of the nose).

There have been only a very few documented instances of patients acquiring HIV from an infected health-care worker.

We have more information about healthcare workers and HIV infection.

Tattoos / piercing

Anything that potentially allows another person's blood to get into your bloodstream carries a risk. If the equipment has not been sterilised before having a tattoo or piercing, there could be a significant risk of exposure if the person before was HIV positive.

In most developed countries there are hygiene regulations governing tattoo and piercing parlours to ensure all instruments used are sterile. If you are thinking of having a tattoo or piercing, ask staff at the shop what procedures they take to avoid infection.