The bacteria, viruses and parasites that cause most sexually transmitted infections (STIs) may be present in the vaginal fluid of a woman who has an STI. In other infections, the bacteria or virus may be present on the skin around the vagina.
If a sexual partner has an STI, it can enter the bloodstream easily if semen or vaginal fluid from the person with an STI gets inside the vagina.
The skin inside the vagina and around the head of the penis is delicate. This means that infections can pass easily between male or female sexual partners during unprotected vaginal sex. The risks increase during longer sex sessions as the skin of vagina or penis may become damaged.
Find out more about sexually transmitted infections (STIs)