The risk of getting HIV from one encounter varies on the type of encounter. You can get HIV through sexual acts, through sharing of needles for drug use/abuse, or through transfusion of blood.

An infected pregnant mother can also pass the infection to her fetus, but this mode of transmission is not what we are discussing here.

You have unprotected sex with someone from either sex, just that once. You realize later that the person is HIV positive, meaning that he/she is infected with HIV (human immunodeficiency virus).

You want to know what are the odds of you getting infected with HIV from exposure to that single unprotected encounter.

You could also have a blood transfusion once or share an injection needle with an infected partner just that once. What are the chances of contracting this viral disease?

The odds vary and depend on certain factors:

  • Whether you were the active or the passive partner
  • The type of sexual act, whether it is vaginal, anal, or oral sex

Type of sexual encounter

The first factor that determines your risk is the type of sexual act you indulged in; whether anal, vaginal or oral and again whether you were the active partner (insertive) or the passive (receptor) partner.

It was and is very difficult to get accurate numbers. But, according to history taken from infected persons, some figures have been arrived at through studies.

Certain facts have been established:

The type of sex you have also has a role in the risk you carry and which can increase your chances of getting the infection.

The following facts tell you the amount of risk of getting HIV from vaginal sex, anal sex, and from oral sex for both the active partner and the passive partner.

Risk of anal sex

Receptive anal sex carries a much greater risk of HIV infection transmission than receptive vaginal sex.

Research indicates that the risk of HIV transmission from receptive anal sex is 18 times higher than transmission from receptive vaginal sex.

According to CDC, for the passive partner in receptive anal sex, the risk is 138 per 10,000 exposures or 1.38 percent

The passive partner in anal sex (receptor) faces a higher risk than the active partner (inserter).

Research suggests the risk of HIV transmission from a single receptive anal sex is 3 to 23 times higher than from insertive anal sex.

The risk from vaginal sex

Receptive vaginal sex is riskier for the woman than insertive vaginal sex by the man. The risk from receptive vaginal sex is about twice as high as that of insertive vaginal sex.

Oral sex

It is unclear exactly how much less risky oral sex is compared to vaginal and anal sex. What is the per-sexual act probability of acquiring HIV from an infected partner?

CDC mentions as “low” the chances of getting HIV from one act of oral sex.

According to, the risk of giving a blowjob to an HIV-positive man, not on treatment is at most 1 in 2,500 (or 0.04 percent per act). Similarly, HIV is present in vaginal secretions so performing oral sex on an HIV-positive woman can be risky.

The risk from various one–time sexual acts

The CDC figures look like this for the type of exposure and the risk of it per 10,000. It should be noted that these figures are for that one-night stand or one-time exposure. Repeated exposures potentially increase the risk.

  • Receptive anal intercourse — chances are 138 per 10,000 – that means, the chances of contracting HIV from one sexual act with an infected partner will be 1.38 percent.
  • Insertive anal intercourse — 11 per 10,000 or 0.11 percent
  • Receptive Penile-Vaginal Intercourse (woman’s risk) — 8 per 10,000 or 0.08 percent
  • Insertive Penile-Vaginal Intercourse (man’s risk) — 4 per 10,000 or 0.04 percent
  • Oral sex, whether receptive or insertive – chances are low.
  • For other modes of HIV transmission such as biting, spitting, or sharing sex toys, the chances are negligible.

HIV risk with sharing of needle one single time 

Sharing a needle one single time with an HIV-infected drug user can carry an HIV risk of 63 in 10,000 or 0.63 percent, which can also be stated as 1 in 149. Certain estimates, however, put the risk as high as 2.4%.

Whenever a syringe is used to inject a drug into the vein, a small amount of blood is initially pulled into the syringe to confirm that the needle is in the vein.

Now, when the same needle is used by another individual, the blood from the previous HIV-positive person that has stayed in the needle can get injected into the bloodstream of an HIV-negative person.

HIV does not generally survive well outside the body, but it can survive for long periods of time (over 28 days) in an airtight syringe.

Besides the intravenous injection route, the risk exists even if the injection is given by the intramuscular route or the subcutaneous route.

HIV risk from a single blood transfusion

Because of careful donor selection criteria, HIV risk from a single blood transfusion has very much reduced due to strict screening measures.

HIV seroprevalence in blood donors is less than 2 percent of the adult American population because of strict screening measures.

Despite this, HIV transmission may still occur due to three reasons:

Blood may be collected during the window period of infection when the donor is infectious but has not yet developed positive results on the HIV laboratory tests.

Infection with variant strains of HIV that may not be detected by regular screening tests.

However, according to CDC,

the risk of HIV transmission by transfusion was low, even before screening, and has been virtually eliminated by the routine screening of donated blood and plasma”.

But, in case HIV-positive blood is given to an HIV-negative individual, the risk of HIV infection with a single transfusion exceeds that of any other risk. More than ninety percent of recipients transfused with HIV-positive blood are found to be infected with the virus.