A pill designed to prevent HIV infection is working better than scientists had expected, according to a new study from Kaiser Permanente San Francisco Medical Center.

Researchers evaluated the effectiveness of the pill Truvada, which is a combination of two anti-HIV drugs — emtricitabine and tenofovir.

The pill is used daily to prevent HIV in people who do not have the disease but are at high risk, and researchers didn't find a single case of new HIV infections among patients during more than two-and-a-half years of observation.

HIV, or human immunodeficiency virus, can lead to AIDs, or acquired immunodeficiency syndrome. Once a person has HIV, they have it for life.

The CDC considers people high-risk who have unprotected sex with people who may have HIV, or those who inject illicit drugs and share needles.

The new study followed 657 patients who were at high risk of contracting HIV. Their average age was 37, and 99 percent were men who have sex with men.

When someone is exposed to HIV, Truvada can keep an infection from becoming permanent.

Although the study was not a clinical trial since all patients received the drug and none were given a placebo, the results were exciting to experts.

During the course of the study, more than half of patients contracted a treatable STI (sexually transmitted infection), indicating condoms weren't always used, but still none of the patients contracted HIV.

The study was published in the journal Clinical Infectious Diseases.