Condoms, when used correctly, prevent the transmission of HIV and other sexually transmitted infections. We now know that people living with HIV who are on treatment and have an undetectable viral load cannot pass HIV to someone else. This is what is meant by U=U (Undetectable equals Untransmittable).  

SEXUAL TRANSMISSION

Different kinds of sex have different risks attached, anal sex is the most risky, followed by vaginal sex.  There is also a risk with oral sex, but the risk is greatly reduced.  HIV is present in sexual fluids - semen, pre-ejaculatory fluid, vaginal fluids and anal mucus and this is why it can be passed on during sex.  Whether HIV is transmitted is dependent on a number of factors, and these can be quite complex.  It is known that viral load is implicated in transmission and the higher the viral load, the higher the risk of transmission.