As much as 16 percent of the U.S. population between the ages of 14 and 49 has genital herpes, according to a government study released Tuesday.

And sexually transmitted diseases of all varieties infect 19 million more Americans every year -- a rate that costs the health care system some $16 billion annually, the National Health and Nutrition Examination Survey (NHANES) found. The study was released by the Centers for Disease Control and Prevention as part of the the National STD Prevention Conference in Atlanta, Georgia.

The herpes simplex 2 virus -- one of the most common sexually transmitted diseases in the United States -- causes painful, recurring genital sores.

"This study serves as a stark reminder that herpes remains a common and serious health threat in the United States. Everyone should be aware of the symptoms, risk factors and steps that can be taken to prevent the spread of this lifelong and incurable infection," said Dr. Kevin Fenton, director of the CDC's National Center for HIV/AIDS, Viral Hepatitis, STD and TB Prevention.

Women and African-Americans have the highest rate of genital herpes infections. According to the study, one in five women is infected with the herpes virus. The infection rate was almost twice as high among women -- nearly 21 percent -- than among men, at 11.5 percent.

For African-Americans, the prevalence of infection was 39.2 percent -- more than three times that of whites, at 12.3 percent. Black women are most affected by the disease, with an infection rate of 48 percent.

Fenton said the high rates of herpes among African-Americans is most likely contributing to the high rate of HIV in that community. In fact, statistics show that people with herpes are two to three times more likely to get HIV if exposed.

The CDC estimates that more than 80 percent of the people who have the herpes simplex 2 virus don't know they are infected. Many people never have symptoms or outbreaks, or those symptoms are mild and mistaken for something else. But transmission can occur without symptoms or visible sores.

"Many individuals are transmitting herpes to others without even knowing it," said Dr. John M. Douglas Jr., director of the CDC's Division of STD Prevention. "We can't afford to be complacent about this disease. It is important that persons with symptoms suggestive of herpes -- especially recurrent sores in the genital area -- seek clinical care to determine if these symptoms may be due to herpes and might benefit from treatment."

These new numbers are from data between 2005 and 2008 and are consistent with the last study, which covered 1999-2004. The CDC says that there was a slight decline in infection rates but that it was not statistically significant.

Prevention strategies can help reduce national rates, the CDC said, and there are a number of effective drugs that can help treat and prevent outbreaks.

Douglas said people who know their herpes status should avoid sex when they have symptoms or sores, limit their number of sexual partners, and use condoms consistently and correctly.

However, CDC experts say there are a number of barriers to treatment, including poverty and lack of access to care. They say young women often don't get tested for STDs because of misinformation and stigma.

According to four studies also released at the National STD Prevention Conference on Tuesday, African-Americans are shouldering a heavy STD burden.

While African-Americans make up 12 percent of the U.S. population, that population accounts for nearly half of all reported syphilis and chlamydia cases, more than 70 percent of all reported gonorrhea cases and about half of all new HIV infections.

And, according to CDC experts, while African-American women and men are often unwilling to get tested, doctors are often reluctant to discuss sexual health with their patients -- particularly with black male patients.