That matters a lot because STIs are more common than you might think. More than half of all sexually active people will contract one at some point. STIs range from the human papillomavirus (HPV) — which you can greatly lower your risks with a vaccine — to gonorrhea, chlamydia, herpes, and HIV, to name just a few.

If your goal is to prevent both pregnancy and STIs, there are only a few ways to make that happen.

Abstinence

Not having sex is the only surefire way to avoid getting pregnant or contracting an STI. Abstaining from sex means different things to different people. For avoiding pregnancy, it means skipping vaginal intercourse. To prevent STIs, it means avoiding oral and anal sex as well.

Outercourse

Outercourse refers to any kind of sexual activity that’s not intercourse. Kissing, touching each other, and dry humping are all forms of outercourse. If semen doesn’t get into the vagina, you won’t get pregnant. But keep in mind that it’s still possible to pick up an STI if you have skin-on-skin genital contact (even if you don’t actually have sex). That’s also true if any of your partner’s fluids get into your mouth or genitals.

Condoms

The only way to be sexually active and prevent both pregnancy and STIs is with a latex or plastic (polyurethane, nitrile, or polyisoprene) condom. You have to use one every time you have sex. Condoms place a physical barrier between you and your partner, which prevents the spread of germs and semen.

Few people use condoms perfectly all the time, but they’re still highly effective. After figuring in breaks and user error, male condoms (worn over the penis) are about 85% effective in preventing pregnancy.

Female condoms, or internal condoms, are also effective at preventing pregnancy and keeping STIs at bay. They’re about 79% effective at preventing pregnancy after accounting for misuse. Just don’t use them at the same time as a male condom or they could break.

A condom is most likely to work properly if you:

• Make sure it isn’t expired.
• Keep it on the entire time you’re having sex.
• Use it with a water-based lubricant, which reduces friction and decreases the chances of it breaking.