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There are four fluids that can transmit HIV: blood, semen, vaginal fluid, and breast milk. These fluids can transmit HIV only if they leave an HIV-positive person’s body and get into another person’s bloodstream. (See our How to Prevent HIV webpage for more about HIV transmission.)

The most common ways that HIV is transmitted are behaviors such as unprotected sex and sharing needles. These are known as "risky behaviors.”Different risky behaviors have very different levels of risk. This depends on things like which body fluid is involved, how much virus is in the fluid, and how the fluid gets into the bloodstream.

It’s important to know how risky different behaviors are. It’s also important to be aware that there is no way to know for sure whether any one act will transmit the virus. There have been many scientific studies done about HIV transmission, but these only give a general idea of risk. There are also other factors that change the risk of a person becoming infected aside from the risky behavior itself, such as having other sexually transmitted infections or other illnesses.

It’s a good idea to educate yourself about how risky different behaviors are. Follow the links below to learn about the risk levels of different behaviors and ways to reduce your risk.

If you have engaged in any risky behavior, the only way to know for sure if you have been infected is to get tested. (See our HIV Testing webpage to find out where to get tested in Philadelphia and beyond.)

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For sexual behaviors sorted by No-Risk, Extremely Low-Risk, Low-Risk, and High-Risk, see HIV InSite’s Safer-Sex Methods webpage.

For a much wider rangeof sexual behaviors, see The Body.com’s archive of questions and answers, HIV Risks From The Body's "Ask the Experts" Forums.

For a more technical discussion of sexual transmission risk, see HIV InSite’s HIV Risks Associated with Specific Sexual Practices.

For the relationship of substance abuse to HIV risk, see the Centers for Disease Control and Prevention (CDC)’s Injection Drug Users / Substance Abusers collection.

To understand how the presence of other sexually transmitted infectionsincreases the likelihood of HIV transmission, see the CDC’s The Role of STD Detection and Treatment in HIV Prevention.

To understand how antiretroviral therapy (AIDS medication) decreases the likelihood of HIV transmission, see AVERT’s HIV Transmission and Antiretroviral Therapy Briefing Sheet.

To understand how circumcisiondecreases the likelihood of HIV transmission, see AVERT’s Circumcision & HIV webpage.

To understand how effective condoms are at decreasing the likelihood of HIV transmission, see the CDC’s Male Latex Condoms and Sexually Transmitted Diseases.

To address common misconceptions about HIV risk, see the CDC’s HIV Transmission webpage and/or AIDS InfoNet’s HIV Myths & Misunderstandings webpage.

To learn more about why risk levels are not an exact science, see:

• AIDS InfoNet’s How Risky Is It? webpage (specifically the "What the Numbers Mean” section)
  
• Columbia Health Services’s Confused About HIV Transmission Statistics Q&A
  
• HIV InSite’s The Risk of HIV Infection Through Receptive Oral Sex roundtable discussion – which focuses on receptive oral sex, but also involves discussion of the challenge of calculating infection risk.

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