STIs are passed on from one person to another by skin-to-skin contact during unprotected oral, anal and vaginal sex. By using condoms and dental dams every time you have sex, you are protecting yourself and your partner against catching or passing on an STI.
Sex can be risky. When you choose to be sexually active, you need to protect yourself by using a condom or a dental dam (which is used for protection when a girl is receiving oral sex). When you use protection during sex, you help stop the spread of STIs (sexually transmitted infections).