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Most people who have HIV got the virus by having unprotected sex with someone who has HIV.¹ Unprotected sex means not using a male or female condom. Researchers don't know whether diaphragms stop HIV infection.¹

HIV lives in a man's semen and a woman's vaginal fluid. If you have unprotected sex with someone who has HIV, the virus can pass from their semen or vaginal fluid into your body through the thin lining of your vagina, anus or rectum, or the surface of your penis.

HIV can also get into your blood through cuts and sores you may have in and around your genitals or anus (some of which you may not know about).

Other factors may also increase the risk of getting HIV through sex. For example, someone who has recently become infected with HIV may be more infectious because they can have a very high level of virus in their blood and semen or vaginal fluid. Also, if you have another sexually transmitted infection, such as gonorrhea or genital herpes, you are two to five times more likely to get HIV than if you didn't have another sexually transmitted infection.² This is because infections can cause breaks such as ulcers or sores in the normal membranes that protect the body's surfaces. These breaks make it easier for the virus to get inside your body.

The lining of the vagina is thin and has lots of blood vessels. So a woman can easily get infected if she has intercourse with an infected man, and neither partner is using a barrier, like a condom. A woman is more likely to become infected with HIV through vaginal sex than a man is.

In anal sex, whether male-male or male-female, both partners are at risk of getting infected. It's more risky than vaginal sex for both partners. The highest risk is to the partner being entered. This is because the lining of the anus and rectum is very thin and filled with many small blood vessels that can be easily damaged.

HIV lives in semen. So if you give unprotected oral sex to a man who is HIV-positive, there's a chance that you'll get infected. One study found that about 3 in 100 men who have HIV probably got it by having oral sex with an infected man.³ Unprotected sex means not using a condom or a dental dam.

The risk is greatest if:

• The man ejaculates into your mouth
• You have cuts or sores in your mouth
• The man has another sexually transmitted disease (STD).

It is difficult to get reliable information on the risk of oral sex as it is rarely the only type of sex people will have. Also, the studies rely on people telling researchers about their sexual activity, which is not always reliable.

The risk of getting infected with HIV during oral sex on a woman is small. But some people have gotten infected in this way.²

You can't tell who has HIV by how they look. A person who has HIV is infectious all the time. The only way to lower your chance of getting HIV from someone who may have the virus is to have "safe sex" or to wait until you know for sure that they don't have HIV.

In safer sex, you create a barrier between you and your partner. The barrier stops your partner's semen or vaginal fluid from getting into your body. Condoms, dental dams (for oral sex) and latex gloves are all examples of barriers used to prevent the spread of HIV.

The risk of becoming infected with HIV for a woman who has vaginal sex with a man who is already infected has been estimated as 0.1 percent for each time they have sex. That's the same as 1 in every 1,000 times you have sex.² The risk for the woman increases by about five times for anal sex.

There hasn't been much research on the risk of HIV infection for women who have sex with women. But HIV is present in vaginal fluids so getting infected in this way is possible.

If you regularly have sex with someone who has HIV, using a condom lowers your risk of getting infected by more than 80 percent.^{4 5} This statistic comes from studies of heterosexual partners.

The more sexual partners you have, the more likely it is that one of your partners will be HIV-positive. You are more at risk if you or your partner has several sexual partners at the same time.

You may think you can have sex without using safer sex practices if you and your partner have tested negative for HIV since the last time you had sex. But you need to be careful. Because the HIV test works by detecting antibodies to HIV, and these can take around six weeks to develop, it is possible to have a negative result even after you've been infected with HIV. This means that before you can think about stopping safer sex practices:

• Both partners must only have sex with each other
• Both partners must have tested negative at least six weeks after the last possible exposure to HIV.

There are treatments for people who think they may have been in contact with HIV. See Emergency treatment if you think you've been exposed to HIV.

This information is for educational use only, and is not a substitute for prompt professional medical advice. Readers should always consult a physician or other professional for advice and treatment. ©BMJ Publishing Group Limited 2008. All rights reserved.