Their lack of awareness contributes to the fact that 40 percent of people with HIV aren't diagnosed until they have developed AIDS, which can be up to 10 years after they were infected. 

That's why health officials around the country are using today to spread awareness.

In New York City, testing centers were set up in Times Square, according to New York 1. In Houston, organizers offered free tickets to a hip-hop concert in exchange for getting tested.

And health officials think measures like these will work. The CDC recently announced findings from a three-year, $111 million initiative they kicked off in 2007 to increase HIV testing awareness. Over three years, 2.8 million tests were given that helped diagnose almost 18,500 people who didn't realize they had HIV.

"But more than half of U.S. adults aged 18-64 still have never been tested for HIV, and our work is far from over." Dr. Kevin Fenton, director of CDC's National Center for HIV/AIDS, viral hepatitis, STD and TB prevention, said in a written statement.