THURSDAY, Jan. 19, 2017 (HealthDay News) -- Many American men are infected with the cancer-causing human papillomavirus (HPV), but unlike women, men are more likely to stay infected throughout their lives, a new study finds.

About 45 percent of U.S. men are infected with the sexually transmitted disease, as are 45 percent of women. Among women, the prevalence of HPV infection drops to about 22 percent as they age, but it remains high among men, said lead researcher Dr. Jasmine Han. She is in the division of gynecologic oncology at Womack Army Medical Center, in Fort Bragg, N.C.

"We don't know why it stays high in men while it drops in women," she said. "Among men it's higher than expected."

Han speculates that the virus may remain in men because it lives in the penile glands, while in women, the virus is near the surface of the vagina and is more easily shed.

Although a vaccine against HPV has been available since 2009, coverage remains low. Only about 11 percent of men and 33 percent of women have been vaccinated, Han said.

HPV is the most common sexually transmitted disease among men and women in the United States, according to background information in the study. About 79 million Americans are infected with some type of HPV, with approximately half of new infections occurring before age 24, the study authors said.

Most people infected with HPV don't know they have it and don't develop health problems from it, according to the U.S. Centers for Disease Control and Prevention.

But HPV is not a benign infection. More than 9,000 cases of HPV-related cancers occur in men each year. HPV is the cause of 63 percent of penile, 91 percent of anal, and 72 percent of oral and throat cancers, the researchers noted.

In addition, HPV among men is an indirect cause of cervical cancer in women. The virus is also responsible for 90 percent of genital warts. HPV can also lead to tumors in the respiratory tract, called respiratory papillomatosis.