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Most people know how HIV is transmitted (see fact sheet 150). They also know about safer sex guidelines (see fact sheet 151). However, they may still be exposed to HIV. This can be by accident or because they take part in some risky behavior. When this happens, they always want to know how likely it is that they got infected with HIV.


You can’t be sure that you’re not infected with HIV unless you are 100% certain that you did not engage in any risky behavior and that you were not exposed to any HIV-infected fluids.

The only way to know for sure whether you have been infected is to get tested. You should wait for 3 months after a possible exposure. Then get an HIV blood test (see fact sheet 102).

You might feel that you have been exposed to HIV by sharing needles, an accident, or unsafe sexual activity. In these cases, talk to your health care provider immediately . Ask whether you can use HIV treatments to prevent infection. Fact sheet 156 has more information on "post-exposure prophylaxis."


In the late 1980s and early 1990s, several studies were done to assess the risks of HIV infection from specific types of exposure to HIV. These calculations only give a general idea of risk. They can tell you which activities carry a higher or lower risk. They cannot tell you if you have been infected.

If the risk is 1 in 100, for example, it doesn?t mean that you can engage in that activity 99 times without any risk of becoming infected. You might become infected with HIV after a single exposure. That can happen the first time you engage in a risky activity.

Also, these studies were based on a specific group of people. There is no reason to believe that the results apply to other groups, or to the general population.


The highest risk of becoming infected with HIV is from sharing needles to inject drugs with someone who is infected with HIV. When you share needles, there is a very high probability that someone else’s blood will be injected into your bloodstream. Hepatitis virus can also be transmitted by sharing needles.

The next greatest risk for HIV infection is from unprotected sexual intercourse (without a condom). Receptive anal intercourse carries the highest risk. The lining of the rectum is very thin. It is damaged very easily during sexual activity. This makes it easier for HIV to enter the body. The "top" or active partner in anal intercourse seems to run a much lower risk. However, the risk still seems higher than for the active partner in insertive vaginal intercourse.

Receptive vaginal intercourse has the next highest risk. The lining of the vagina is stronger than in the rectum, but is vulnerable to infection. Also, it can be damaged by sexual activity. All it takes is a tiny scrape that can be too small to see. The risk of infection is increased if there is any inflammation or infection in the vagina.

The risk is higher for the receptive partner. However, there is some risk for the active partner in anal or vaginal sex. It’s possible for HIV to enter the penis through any open sores, through the moist lining of the opening of the penis, or through the cells in the mucous membrane in the foreskin or the head of the penis.


There have been many studies of HIV transmission through oral sex. They have not come to clear conclusions. However, the following points are clear:

  • It is possible to get infected with HIV through oral sex. The risk is not zero.
  • The risk of HIV infection through oral sex is extremely low. It is much lower than for other types of unprotected sexual activity. However, other diseases such as syphilis can be transmitted through oral sex.


Syphilis can increase the risk of transmitting HIV. People with syphilis have a higher than average chance of being infected with HIV. Also, syphilis causes large, painless sores. It is easy for someone to be infected with HIV through syphilis sores. Herpes simplex infection (see Fact Sheet 508) also causes sores which assist infection with HIV. An active case of syphilis or herpes increases the amount of HIV in someone’s system and can make it easier for them to pass it on to another person.

Several other factors increase the risk of transmitting HIV, or becoming infected.

  • When the HIV-infected person is in the "acute infection" phase (see fact sheet 103), the amount of virus in their blood is very high. This increases the chance that they can pass on the infection. Unfortunately, almost no one knows when they are in this phase of HIV infection. There’s no way to tell by looking at them.
  • When either person has a weakened immune system. This could be because of a long-term illness or an active infection like a herpes outbreak, syphilis, or the flu.
  • When the uninfected person has open sores that get exposed to infected fluids. These could be cold sores, genital herpes, mouth ulcers, syphilis sores, or other cuts or breaks in the skin.
  • When there is exposure to infected blood.
  • When the uninfected insertive male partner is not circumcised.


Researchers have developed estimates of the risk of transmission of HIV. These estimates can give you a general idea of which activities are more or less risky. They cannot tell you that any activity is safe, or how many times you can do them without getting infected. The best way to avoid infection is to use a condom correctly and consistently for all sexual activity, and to avoid sharing needles. If you think you have been exposed to HIV, wait 3 months and get tested.

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