FRIDAY, Dec. 5 (HealthDay News) -- New research is helping to unravel the mystery of how a small number of HIV-infected people manage to keep the AIDS-causing virus at bay without needing to take medicine.

The key seems to be an unusual feature of the immune systems of these fortunate patients, and researchers hope it might lead to better drug therapies or even vaccines.

So-called "non-progressors" -- people who get infected with HIV but don't get sick -- appear to have super-powered immune cells, according to a study published in the Dec. 4 issue of Immunity. Essentially, these cells gain more killing power through extra ammunition, explained study co-author Dr. Mark Connors, a researcher at the U.S. National Institute of Allergy and Infectious Diseases.

"It was not just a numbers thing," Connors said. Instead, the killer cells were actually better equipped to kill off the virus in the AIDS-infected patients who didn't get sick.

Researchers have long been fascinated by these patients, some of whom have avoided becoming sick for 20 years or more. The AIDS virus is definitely in their bodies: They are diagnosed by AIDS tests and other tests show the germs are there.

"These people do have HIV in their system, and their immune system is fighting the virus," said Rowena Johnston, vice president of research for the Foundation for AIDS Research.

But the virus remains at extremely low levels in their body, sometimes even lower than in ordinary AIDS patients who successfully take powerful medications.

Experts estimate that 1 percent or less of AIDS patients are non-progressors. Still, the idea that anyone could get infected with the virus and keep it under control is amazing to scientists.

For the new study, Connors and his colleagues looked at immune cells from normal AIDS patients and non-progressors to see how they reacted to the AIDS virus. The researchers are trying to understand how the immune systems in non-progressors hold the virus in check over a long period of time.