STIs are transmitted through sexual activities such as oral, vaginal or anal sex. Some STIs can also be spread through contact with infected blood. STIs are contagious which means you can contract them from your partner(s) if they are infected. STIs will stay with you if you do not receive treatment for them. STIs are most commonly found in the rectum (bum), in or around the vagina, in and around the penis and/or in the mouth.