The data, drawn from a sample of 838 girls who participated in a broad national survey in 2003-4, was presented last week by researchers from the Centers for Disease Control and Prevention. By far the most common of the four S.T.D.'s was the human papillomavirus, or HPV, which infected 18 percent of the girls. Chlamydia infected 4 percent, trichomoniasis - a common parasite - 2.5 percent, and genital herpes 2 percent.

The study did not look at such feared diseases as H.I.V./AIDS, syphilis or gonorrhea, but the four it did look at are worrisome enough. Although most HPV infections cause no symptoms and clear the body in less than a year, persistent HPV can cause cervical cancer and genital warts. S.T.D.'s can cause infertility, pelvic inflammatory disease and other painful symptoms.

It will not be easy for sexually active teenagers to avoid any danger. Even among girls who said they had had only a single sexual partner, 20 percent were infected. With more than three million teenage girls infected, it is imperative to find ways to protect others.

The new findings strengthen the case for providing HPV vaccine to young girls and for regular screening of sexually active girls to detect infection. There is also a clear need to strengthen programs in sex education. Exhortations to practice abstinence go only so far.

Teenage girls who are sexually active need access to contraceptives and counseling. They need to understand that the numbers are against them and that a serious infection is but a careless sexual encounter away.

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