Nearly half a million Americans may have the sexually transmitted infection (STI) chlamydia without knowing it, according to the U.S. National Center for HIV/AIDS, Viral Hepatitis, STD, and TB Prevention. Researchers at the center analyzed data from the 2007 to 2010 National Health and Nutrition Examination Survey and presented their findings at the STD Prevention Conference in Atlanta.

Chlamydia and other STIs can facilitate the transmission of HIV, both by making people living with HIV more infectious and making those without the virus more susceptible to acquisition.

The investigators found that 1.7 percent of Americans between 14 and 39 years old have chlamydia. This translates to around 1.8 million infections per year in the United States. Considering that only about 1.4 million infections are reported annually, 400,000 people may go undiagnosed.

The rate of infection is the highest among sexually active girls between 14 and 19 years old, 6.4 percent of whom have chlamydia. (Teen girls' cervex's aren't yet fully mature, making them more susceptible to the infection.) Boys in that age bracket, by comparison, have an infection rate of 2.4 percent. Sexually active black teen girls have an infection rate of 18.6 percent, compared with 3.2 percent among their white teen-girl counterparts.

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