Pre-exposure prophylaxis, or PrEP, is a proven method to prevent HIV infection, but less than 3% of gay men that meet high-risk criteria are currently on Truvada.

A new report from the CDC indicates that some 1.2 million gay and bisexual men should be on PrEP, or about one in four. Currently, only an estimated 21,000 people nationwide are.

Though no method is foolproof, the once-a-day drug reduces the likelihood of infection via sexual contact by 92%. In a study from September, researchers found that 100% of participants remained HIV-free after taking Truvada for two years.

"PrEP isn't right for everyone. No single method is, but it's right for some people, and when the men and women at high risk adhere to PrEP or whatever prevention methods work for them, we can make gains in national efforts," said Eugene McCray, director of HIV/AIDS prevention at the CDC.

Only approved by the FDA in 2012, Truvada was initially only suggested for a small segment of the MSM population. It's now been recommended for wider use by the CDC, World Health Organization and other groups.

New cases of HIV are declining across the board—except among young men who sleep with men. Their numbers are sadly spiking: Between 2002 and 2011, the number of new infections among gay and bi men ages 13 to 24 more than doubled.

Below are the CDC guidelines for individuals considered at substantial risk of HIV infection who should consider PrEP:

* Sexually active adults in an ongoing relationship with an HIV-infected partner.

* Gay and bisexual men who aren't in a monogamous relationship and don't always use condoms or have been recently diagnosed with a sexually transmitted infection.

* People who inject drugs that aren't prescribed by a clinician.

* Heterosexual men and women who don't always use condoms with sexual partners who are at substantial risk for HIV infection.