Parents may hope their teenage daughters are spending time on innocent pursuits like shopping, socializing and playing sports. But a recent study by the Centers for Disease Control and Prevention reveals that’s not always the case.

The study finds that at least half of all teen girls in the United States are sexually active and one in every four is infected with a sexually transmitted disease. Researchers uncovered these facts by testing more than eight-hundred girls between the ages of fourteen and nineteen for human papillomavirus [pap-ih-LOH-mah-virus], Chlamydia [klah-mid-ee-uh], trichomoniasis [trick-oh-moh-nye-ah-siss] and herpes. They discovered that half of African-American teen girls — and one in every five white girls — are infected with at least one disease.

The most common infection was human papillomavirus, the sexually-transmitted virus that can cause genital warts and cervical cancer. The virus often goes undiagnosed because it doesn’t always cause external symptoms. The C-D-C and the American Academy of Pediatrics recommends vaccinating girls against H-P-V by age twelve, before they become sexually active.

Each of the other S-T-Ds were much less common. Sexually active women under the age of 25 are encouraged to undergo annual chlamydia screenings. Most cases of chlamydia and trichomoniasis can be easily cleared with antibiotics.

The study is a wake-up call to both parents and teens alike. Experts say parents should remember that communication is the key to keeping kids healthy throughout the tumultuous teen years. And teens, remember: Staying smart about sex means staying safe.