STIs, also known as STDs, are infections passed between people through sexual contact. They can be caused by viruses or bacteria. Common STIs include chlamydia, gonorrhea, herpes, HIV, and HPV. STIs spread through unprotected vaginal, oral, or anal sex with an infected partner. Women are often at higher risk than men because female anatomy makes transmission easier and some STIs may not show obvious symptoms. The best ways to prevent STIs include getting vaccinated, using condoms consistently, getting tested regularly, being in a mutually monogamous relationship, limiting number of partners, and avoiding alcohol and drug abuse.