But sometimes the evolving virus can unlock a response that holds HIV in check. Levy told Brothers he had a drop of luck in his blood. His white blood cells seemed to secrete tiny amounts of a substance that controls HIV. At the time, Brothers was only one of several hundred people, out of tens of millions with HIV, known to control HIV in this way. Levy believes an unidentified protein is responsible, and isolating and harnessing it might allow scientists to produce a revolutionary HIV treatment.