Condoms can help protect you from sexually transmitted infections like HIV, gonorrhea, chlamydia, and high-risk HPV — but they're not fail-safe, and they don't protect against everything. "Condoms are not effective at preventing the transmission of herpes or warts, since those lesions can happen on an area not covered by the condom," Dr. Yamaguchi says. That's why STI testing is super-important if you're sexually active — yep, even if you're being careful.