It difficult to determine the exact chance of something passing as there are so many different factors. For example, if she does not have HIV there is no risk, if she has HIV and is taking HIV medication with an undetectable viral load there would be very little risk.
When performing oral sex on a vagina there is a chance HIV could pass to someone as HIV can be in the vaginal fluids and get into your body via the mouth. The chances of this happening is low as we find it is not common for HIV to pass this way.
As most sexually transmitted infections including HIV do not show symptoms the only way to know if someone has HIV is by going for a HIV test.
Have a look at our “Know your chances” web page, it provides information on what STI pass with oral sex and your chances of getting or passing an STI depending on how you are having sex.
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