Fewer Americans are engaging in risky behaviors that could lead to being infected with HIV, according to a national health report issued Thursday. At the same time, another report found that the incidence of two sexually transmitted diseases — gonorrhea and chlamydia — increased in 2010.

When researchers at the Centers for Disease Control and Prevention looked at risky behaviors involving sex or drug use, both linked to transmission of HIV, they found a decrease from 13 percent to 10 percent in men from 2002 to 2010, and in women a drop from 11 percent to 8 percent.

By the end of 2008, CDC says, about 1.2 million people in the United States were living with HIV, which is the precursor to AIDS, about 1 in 5 of them without knowing it. And an estimated 48,100 were newly infected in 2009.

The national estimate was based on 22,682 face-to-face interviews with people ages 15 to 44 in their homes.

The sexually risky behaviors linked to HIV infection included homosexual encounters, multiple sexual partners, sex in exchange for money or drugs and having a sexual partner who injects illicit drugs or who is HIV positive. Shared needles is another red-flag behavior.

"Generally, these are behaviors that are studied in higher-risk populations, but by looking in the household population we can get a better sense of the level of risk that may exist in the general population that you don't normally think about," report author Anjani Chandra of the CDC's National Center for Health Statistics told HealthDay.

She said the reasons for the decline were unclear, but it could be that people were better informed about risk than in the past. It is also possible, she told HealthDay, that people might be reluctant to admit to risky behaviors.

While health experts were hailing that decline, others were worried about different, troubling STD trends. In another report released this week, the CDC noted increases in two of the three sexually transmitted diseases where reporting is required, gonorrhea and chlamydia, while the overall syphilis rate is down slightly but it has increased in at least one demographic. The three STDs "represent only a fraction of the true burden of STDs. Some common STDs like human papilloma virus and genital herpes are not required to be reported," the report noted.

It said that STDs are "one of the most critical health challenges facing the nation today. CDC estimates that there are 19 million new infections every year in the United States." The cost, it said, is $17 billion, and the burden on individuals is even greater in "immediate and lifelong health consequences."

In 2010, 1.3 million cases of chlamydia infection were reported, the "largest number of cases ever reported to CDC for any reason." It represented a 4.1 percent increase compared to the rate in 2009. But the report suggested that the increase "most likely represents a continued increase in screening for this usually asymptomatic infection, expanded use of more sensitive tests and more complete national reporting." It also noted that fewer than half of sexually active young women are screened according to CDC recommendations.

Gonorrhea cases increased slightly after years of very dramatic decline, the CDC found. Between 1975 and 1996, it fell 74 percent, then plateaued for a decade before dropping again between 2006 and 2009 to its lowest rate ever. In 2010, it increased 2.8 percent with a total of 209,341 cases reported in the United States. One of the biggest issues with gonorrhea, the report noted, is the possibility of antimicrobial resistance, which would mean that the only available treatment would not work.

Syphilis fell precipitously from 1990 to 2000, by nearly 90 percent, before starting an annual climb through 2009. The 2010 report year was the first year for new decreases overall, down 1.6 percent. but it is climbing in some populations, particularly among young black men, the group noted.

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