Startling government research on teenage girls and sexually transmitted diseases sends a blunt message to kids who think they’re immune: It’s liable to happen to you or someone you know.

In the first study of its kind, researchers at the federal Centers for Disease Control and Prevention found at least one in 4 teenage American girls has a sexually transmitted disease.

The most common one is a virus that can cause cervical cancer, and the second most common can cause infertility. Nearly half the black teens in the study had at least one sexually transmitted infection, versus 20 percent among both whites and Mexican-American teens.

The study, released Tuesday at an STD prevention conference, has adolescent-health specialists pointing to possible reasons and offering potential solutions.

Blame is most often placed on inadequate sex education, from parents and from schools focusing too much on abstinence-only programs. Add to that a young person’s sense of being invulnerable.

“This is pretty shocking,” said Dr. Elizabeth Alderman, an adolescent medicine specialist at Montefiore Medical Center’s Children’s Hospital in New York.

“To talk about abstinence is not a bad thing,” but teen girls — and boys too — need to be informed about how to protect themselves if they do have sex, Alderman said.

Only about half of the girls in the study acknowledged having sex. Some teens define sex as only intercourse, yet other types of intimate behavior including oral sex can spread some diseases.

Among those who admitted having sex, the rate was even more disturbing — 40 percent had an STD.

“Those numbers are certainly alarming,” said sex education expert Nora Gelperin, who works with a teen-written Web site called sexetc.org.

“Sexuality is still a very taboo subject in our society,” she said. “Teens tell us that they can’t make decisions in the dark and that adults aren’t properly preparing them to make responsible decisions.”

Cecile Richards, president of Planned Parenthood Federation of America, said the study shows that “the national policy of promoting abstinence-only programs is a $1.5 billion failure, and teenage girls are paying the real price.”

Similar claims were made last year when the government announced the teen birth rate rose between 2005 and 2006, the first increase in 15 years.

The overall STD rate among the 838 girls in the study was 26 percent, which translates to more than 3 million girls nationwide, the CDC said.

HPV most common

The study by CDC researcher Dr. Sara Forhan is an analysis of nationally representative data on 838 girls aged 14 to 19 who took part in a 2003-04 government health survey. Teens were tested for four infections: human papillomavirus, or HPV, which can cause cervical cancer and affected 18 percent of girls studied; chlamydia, which affected 4 percent; trichomoniasis, 2.5 percent; and genital herpes, 2 percent.

Dr. John Douglas, director of the CDC’s division of STD prevention, said the results are the first to examine the combined national prevalence of common sexually transmitted diseases among adolescent girls. He said the data likely reflect current prevalence rates.

HPV can cause genital warts but often has no symptoms. A vaccine targeting several HPV strains recently became available, but Douglas said it likely has not yet had much impact on HPV prevalence rates in teen girls.

Chlamydia can cause an abnormal discharge and painful urination, but often has no symptoms. Signs of trichomoniasis are similar, and both diseases can be treated with antibiotics. Genital herpes can cause blisters but also is often symptomless. It can’t be cured but medicine can help.

The CDC recommends annual chlamydia screening for all sexually active women under age 25. It also recommends the three-dose HPV vaccine for girls aged 11-12 years, and catch-up shots for females aged 13 to 26.

The CDC’s Dr. Kevin Fenton said that given the potential complications from STDs, “screening, vaccination and other prevention strategies for sexually active women are among our highest public health priorities.”

Douglas said screening tests are underused in part because many teens don’t think they’re at risk, but also, some doctors mistakenly think: “Sexually transmitted diseases don’t happen to the kinds of patients I see.”

Teens need to hear the dual message that STDs can be prevented by abstinence and condoms — and hear them often, said Dr. Ellen Kruger, an obstetrician-gynecologist at Ochsner Medical Center in New Orleans.

“You’ve got to hammer at them,” with appropriate information at each stage of teen development to make sure it sinks in, she said.

She said there are a lot of myths out there, too — many sexually active teens think the withdrawal method will protect them, or that douching with Coca-Cola will kill STD germs.

Dr. Margaret Blythe, an adolescent medicine specialist at Indiana University School of Medicine, said some doctors hesitate to discuss STDs with teen patients or offer screening because of confidentiality concerns, knowing parents would have to be told of the results.

Blythe, who heads an American Academy of Pediatrics committee on adolescence, noted that the academy supports confidential teen screening.