While out with friends in Los Angeles last Spring, Cody Woods found himself sharing what he learned about ways to avoid contracting HIV. A few weeks earlier, he had been immersed in another discussion on Facebook about sexually transmitted diseases, this time correcting something a friend had posted about Hepatitis B transmission.

“I remember one of my friends posting [on Facebook] about Hep B and that you can get it through making out with someone,” says Woods, 22. “I commented that ‘actually it…