SATURDAY, June 14 (HealthDay News) -- More than half of all people will have a sexually transmitted disease or infection at some point in their life, the American Social Health Association reports.

One of the least noticeable, but potentially most life-threatening infections, is the human papillomavirus, or HPV.

Most HPV carriers are never diagnosed and never realize they carry the virus.

"It's never detected, they are never aware of it, and their immune system suppresses it before they ever know about it in the vast majority of cases," said Fred Wyand, spokesman for the American Social Health Association.

In this way, HPV is a silent killer. It's the leading cause of cervical cancer and has become the second-leading cause of cancer death for women around the globe.

Doctors have responded to the threat of HPV by fighting it in a way unusual among sexually transmitted diseases -- through a vaccine. The vaccine, Gardasil, is proven to prevent infection from four particularly dangerous strains of HPV in women. The U.S. Centers for Disease Control and Prevention has recommended that 11- and 12-year-old girls begin receiving the vaccine as part of school vaccination efforts.

Now researchers are looking into whether the vaccine should be given to boys as well, both to prevent the transmission of HPV, and to prevent the rarer, but no less deadly, cancers that can occur in men from the virus.

"There is probably no reason to think it would not be effective in boys, and because HPV is passed back and forth, immunizing a large part of the population would limit transmission," said Dr. Jonathan L. Temte, associate professor in the Department of Family Medicine at the University of Wisconsin School of Medicine and Public Health.

"However, we're still very early in the life span of this vaccine. It's been less than a year since a recommendation was issued. It is premature to discuss giving it to boys until there's proof of its efficacy," added Temte, who also serves as the American Academy of Family Practitioners' liaison to the Advisory Committee on Immunization Practices.

Studies have shown HPV to kill an estimated 240,000 women worldwide each year through cervical cancer. And infection with HPV via oral sex also has been proven to be the leading cause of throat cancer, striking 11,000 American men and women each year.

Research continues to find that Gardasil is very effective in preventing HPV-caused cervical cancer. Two studies last year involving almost 18,000 girls and women found that Gardasil was nearly 100 percent effective in preventing precancerous cervical lesions from the four HPV strains targeted by the vaccine. Though there are at least 15 strains of genital HPV, Gardasil targets the four strains thought to cause 70 percent of cervical malignancies.

The studies also found that Gardasil is much more effective when given to girls or young women before they become sexually active.

Although men don't risk cervical cancer, they are half of the equation when it comes to sexually transmitted diseases. They also face increased risks for throat, genital and anal cancers from HPV infection.

The maker of Gardasil, Merck & Co., is accumulating data to consider whether boys should receive the inoculation as well.

"Nobody will be surprised if someday it is recommended for boys, but it's premature to make that call now," Wyand said. "The early returns I'm aware of with boys are positive. The vaccine appears to trigger an immune response similar to that of girls."

Gardasil isn't the only development on the vaccine front -- other vaccines for sexually transmitted diseases are being studied as well, Temte said. A second HPV vaccine, this one from GlaxoSmithKline, is currently awaiting FDA approval, he said.

And researchers are also looking at a vaccine that could prevent herpes simplex, the cause of genital herpes. "There are going to be a few years out before we see anything like that," Temte said.

Other news involving sexually transmitted disease is less encouraging.

The CDC estimates that approximately 19 million new sexually transmitted infections occur each year, almost half of them among young people ages 15 to 24. Direct medical costs associated with STDs consume up to $14.7 billion annually in the United States.

And, in 2006, there were increases in chlamydia, gonorrhea, and syphilis in the United States, according to the CDC.

More than 1.03 million cases of chlamydia were reported in 2006, up from 976,445 in 2005. Gonorrhea has increased for two years in a row, following a 74 percent decline in its reported rate for two decades. And the national syphilis rate increased 13.8 percent between 2005 and 2006, again reversing what had been years of decline.

Doctors are investigating what these increases mean, Wyand said.

"They aren't sure if those were true increases, or if people are being tested with better and more specific technologies," he said, noting that each of the STDs tend to be chronically underreported.

More information

To learn more about sexually transmitted diseases, visit the American Social Health Association.