A sexually transmitted infection (STI) or sexually transmitted disease (STD) is an infection or disease that spreads from one person to another during sexual contact. This can be through vaginal, oral or anal sex, sharing of sex toys, or skin-to-skin contact.
Sometimes, STIs can also be transmitted nonsexually (e.g., through contact with infected blood or bodily fluids, sharing needles, or from mother to child during pregnancy or childbirth). STIs can be caused by bacteria, viruses, or parasites.