Most STIs are passed on through body fluids such as semen (including pre-cum), vaginal fluids, anal mucus, or blood.
Sex without a condom is the most common way for STIs to be passed on. This could be vaginal sex (when a penis enters a vagina) or anal sex (when a penis enters the bottom).
STIs can be passed on through oral sex (see below). The risk of getting an STI from oral sex is usually much lower than for vaginal or anal sex.
STIs can also be passed on through sharing sex toys and some can be passed on through skin-to-skin contact.
Blood-borne viruses such as HIV and hepatitis can be passed on in a few other ways, most commonly through sharing drug-injecting equipment.
Many STIs can be passed on to a baby during pregnancy. This is why it’s a good idea to find out about your sexual health before trying for a baby (see below)