FRIDAY, Feb. 3, 2017 (HealthDay News) -- About 2.5 million American women have had pelvic inflammatory disease, an often-symptomless infection of the reproductive tract that can cause infertility and lasting abdominal pain, a new U.S. government report shows.

Scientists also found that those who had 10 or more male sex partners experienced pelvic inflammatory disease three times as often as women with a single partner.

The culprits in most cases of pelvic inflammatory disease, or PID, are the sexually transmitted diseases (STDs) chlamydia and gonorrhea, researchers said. Overall, about 4.4 percent of American women said they'd been diagnosed with PID.

Pelvic inflammatory disease tends to be a later "complication of having a prior sexually transmitted infection, and given that there are such high numbers of chlamydia and gonorrhea infections in the U.S., it means a lot of women are at risk," said study author Kristen Kreisel. She's an epidemiologist with the Division of STD Prevention at the U.S. Centers for Disease Control and Prevention.

"Pelvic inflammatory disease can be treated, as well as the STD that caused it, but the structural damage that occurs because of PID is often irreversible," Kreisel said. "That's why it's important to stay on top of it."

Chlamydia and gonorrhea are the most commonly reported STDs in the United States. About 1.5 million chlamydia and 400,000 gonorrhea infections were reported in 2015. These infections often don't cause symptoms and may go undiagnosed and untreated, according to the study.

When symptoms do appear, they may include abnormal vaginal discharge or a burning sensation during urination.

Pelvic inflammatory disease symptoms may include persistent abdominal pain, fever, abnormal vaginal discharge, or pain or bleeding during sexual intercourse, Kreisel said.

PID poses long-term hazards such as infertility, chronic pelvic pain and ectopic pregnancy. Ectopic pregnancies occur when an embryo implants in the fallopian tube instead of the uterus.

No single test can diagnose PID, so doctors often rely on symptom reporting. But PID often doesn't cause symptoms, Kreisel explained. "That's one of the scariest things because you may not know it's even happening," she said.