if you have sex with someone who has HIV are you guaranteed to catch HIV?
No, the opposite is true, HIV is a difficult virus to catch.
The actual risk in any single situation probably vary from zero (no risk) to to less than 1 in 20,000 (0.05% risk per exposure) to 1 in 10 (10% risk per exposure). This will depend on many things, including: the viral load of the HIV positive partner, the type of sex, whether you use a condom, if there are STIs, genetics etc).
The risk is zero if the positive partner is on treatment (ART) with an undetectable viral load. An undetectable viral load makes HIV untransmittable. (See: The evidence for U=U).
Low risks (1 in 5,000) include oral sex generally (not in early infection), or perhaps the risk of a condom breaking.
High risk – perhaps 1 in 100 to 300 – is estimated from vaginal or anal intercourse without using a condom (not accounting for viral load), or for oral sex with someone who has recently been infected.
Highest risk would involve blood to blood contact with someone who has a high viral load.
Although HIV is not an easy virus to catch – it does only takes one exposure for infection to occur in any risk setting other than zero. There is probably zero risk if the positive partner has undetectable viral load, if the negative partner uses PrEP or when condoms are used properly.
Many HIV positive people have sex with HIV negative people, including in long-term relationships, without the negative person catching HIV.
See also the info and links from this page.
This question was updated in January 2018 from an original answer posted in May 2008. i-Base no longer answers individual questions about HIV transmission and risk. (See: Question 1, 5, 6 and 9 at this link).
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