As it stands, it's hard to give a sensible answer to that question. We don't know anything about the things which would make acquiring HIV more or less likely, like the sexual activities that were involved, but we do know that it was a "one night stand".
Let's start by scrubbing the "one night stand" bit from the question. In terms of HIV, it's completely irrelevant whether sex took place as a one-off or in a ten-year relationship, in a brothel or a marital bed, with someone you love or with someone you regret ever meeting.
But there is a reasonable question to be asked about the odds of getting HIV during a single sexual act.
To answer it, we need to know whether the person you're having sex with has HIV or not. After all, the odds of getting HIV from a person who doesn't have it are absolutely zero.
To answer it, we also need to know what kind of sex we are talking about. Let's limit this discussion to penetrative vaginal or anal sex. (Suffice to say that the HIV risks from other sexual activities are much much lower).
And we need to know whether the HIV positive person you're having sex with is taking HIV treatment or not. Because somebody who is taking effective HIV treatment and has what is known as an "undetectable viral load" is extremely unlikely to pass on HIV.
It's also vital to know whether a male or female condom was used. These are as effective as HIV treatment in stopping transmission of the virus.
So let's say you had penetrative sex with somebody who has HIV but isn't receiving medical treatment for it, and you didn't use a condom. For vaginal sex or for anal sex as the insertive partner, the odds may be about one in 1000. For anal sex as the receptive partner (bottom), the odds may be about one in 100.
So HIV wouldn't be transmitted on each and every occasion, but you will never know which occasion could be the unlucky one.
More on HIV Transmission Risks at TheBody.com
To find out more about how HIV is passed on, we recommend the following articles:
In addition, our Q&A experts sometimes address questions about one-night stands in our "Ask the Experts" forums. Here are some of those questions and our experts' responses:
- unprotected one night stand
I am 23 and I'm in a long-term relationship with my girlfriend. I had a one-night stand, Including unprotected sex -- it was a drunken mistake. What are my chances of having contracted HIV or an STD?
- one night stand, should I worry?
I have never done it with a stranger before so I was worried. So I called her and took her to the hospital for HIV testing and she tested negative. I am still worried.