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HIV is a virus that weakens your immune system by killing CD4 cells, which are a type of immune cell. People living with untreated HIV are more vulnerable to disease.

HIV can progress into AIDS if left untreated. The World Health Organization estimated at the end of 2020 that about 37.7 million people are living with HIV worldwide.

HIV is transmitted between people through bodily fluids. Unprotected anal or vaginal sex and sharing needles are common ways people contract HIV. People can also transmit HIV to their babies during pregnancy or while breastfeeding.

HIV can be transmitted even with a single, brief exposure. The authors of a 2014 review of studies estimated chances of infection for various types of exposures. Based on their data, the table below shows the estimated risk of contracting HIV for a single exposure event:

Exposure typeEstimated infections per 10,000 exposuresEstimated risk for a single exposure
Blood transfusion9,25092.5%
Needle-sharing630.6%
Receiving anal sex1381.4%
Insertive anal sex110.1%
Receiving penile-vaginal sex80.1%
Insertive penile-vaginal sex4<0.1%

Various factors can influence your chances of contracting HIV after exposure. Keep reading as we take a look at these factors and what you should do if you think you’ve been exposed.

HIV passes between humans through bodily fluids. The following are the most common ways it’s transmitted.

Sex without a condom or barrier method

HIV can be transmitted through:

  • blood
  • semen
  • pre-seminal fluid (pre-ejaculate)
  • rectal fluids
  • vaginal fluids

It’s possible to contract HIV when any of these fluids from a person living with HIV come into contact with your mucus membranes, such as those found in your:

  • mouth
  • vagina
  • penis
  • rectum

Sharing needles

Needles can transmit small amounts of blood from person to person, even if no blood is visible on the syringe. Sharing syringes also increases your risk of developing other diseases such as hepatitis B or C.

HIV and pregnancy

HIV can be transmitted from parent to child during pregnancy, during delivery, or through breast milk. Parent-to-child transmission is the most common way children get HIV.

There are ways a pregnant person living with HIV can avoid transmitting the virus to their child. According to the CDC, if a pregnant person takes HIV medication throughout pregnancy and childbirth, and the baby is given HIV medication for 4 to 6 weeks after birth, the risk of transmission is less than 1 percent.

The following factors influence your chances of developing HIV after exposure.

Viral load

Viral load is a measure of how much of a virus is in a person’s blood. The higher somebody’s viral load, the higher their chances of passing HIV to another person. People taking HIV medication can suppress HIV to the point that it’s undetectable after 7 to 12 months and therefore untransmittable.

Multiple exposures

Having multiple exposures to HIV increases your risk of infection. For example, if you engage in sex without a condom or other barrier method with an HIV-positive person multiple times, your risk of contracting HIV increases.

Multiple sexual partners

Having multiple sexual partners increases the odds that one of your partners will have HIV.

Having an STI

You’re more likely to transmit HIV when you have genital ulcers or urethritis.

Having other sexually transmitted infections (STIs) like gonorrhea or syphilis is suggestive of having sex without a barrier method with at least one sexual partner. Having sex without a barrier method increases your risk of developing an HIV infection.

People unaware of having HIV

It’s estimated that about 1 in 7 people living with HIV in the United States don’t know they have the virus.

People who are unaware that they have HIV are less likely to take precautions to avoid transmission to other people. They also likely don’t take medications to suppress the virus.

If you don’t currently have HIV, you can prevent your chances of infection by:

  • discussing HIV and STIs with your partner before engaging in sexual activity
  • using a barrier method every time you engage in sexual activity
  • avoiding sharing needles
  • talking with your doctor about postexposure prophylaxis (PEP) if you may have been exposed to HIV in the past 72 hours
  • getting tested for other STIs regularly or before engaging in sexual activity with a new partner

If you do have HIV, you can prevent transmitting it to others by:

  • discussing HIV and STIs with your partner before engaging in sexual activity
  • using a barrier method every time you engage in sexual activity
  • taking your medications as prescribed
  • avoiding sharing needles or drug injection equipment
  • having your viral load tested regularly as recommended by your doctor

Preexposure prophylaxis (PrEP)

Preexposure prophylaxis (PrEP) is a drug that reduces your chance of developing HIV from sexual activity by about 99 percent and from injected drugs by about 74 percent when used as prescribed. If you plan to take this drug, you will need to see how long to take it before it becomes effective.

If you think you’ve been exposed to HIV, here’s what you can do.

Talk with a medical professional

A medical professional can advise you on what tests are available and let you know if you’re eligible for PEP.

Testing

It’s important to get tested if you think you may have been exposed to HIV so you can start treatment as soon as possible.

HIV tests can’t immediately detect infection. The window period of when a test can correctly identify a positive infection varies by the type of test used, but it’s at least 10 days after exposure.

Postexposure prophylaxis

Postexposure prophylaxis (PEP) is a drug therapy that needs to be taken within 72 hours of exposure, and it can significantly reduce your chances of developing HIV.

The Centers for Disease Control and Prevention (CDC) recommends that everybody between the ages of 13 to 64 get tested for HIV at least once. It’s also critical to get tested after potential exposures.

Types of tests

There are three types of tests used to diagnose HIV:

  • Nucleic acid test (NAT). This test looks for the virus itself in your blood. It’s used for people who have early symptoms or have had a high-risk exposure.
  • Antibody test. An antibody test is usually performed with a blood draw, finger prick, or with oral fluids. The test searches for antibodies produced by your immune system to protect against HIV.
  • Antigen/antibody test. This is usually performed with a blood draw or a finger prick. It searches for both antibodies and antigens, which are proteins on the surface of the virus.

When to test

HIV tests are only accurate once a certain number of days have passed after exposure. The minimum time that needs to pass for each type of test is:

  • NAT: 10 to 33 days
  • Antibody test: 23 to 90 days
  • Antigen/antibody test: 18 to 45 days for a blood draw or 18 to 90 days for finger prick

How often to test

If your test comes up negative, you should test again at the end of your window period.

According to the CDC, if you test negative again after your window period and you didn’t have another possible exposure during your window period, then you don’t have HIV.

If you think you may have been exposed to HIV, it’s a good idea to visit a healthcare professional as soon as possible. They can help walk you through the testing process, let you know if you’re eligible for postexposure prophylaxis, and answer any specific questions you may have.

The chances of developing HIV after exposure depends on factors such as the method of exposure, the infected person’s viral load, and the number of exposures you had. It’s possible to develop HIV after a single exposure, even if it’s statistically unlikely.