Treatment is part of a total effort to contain the spread of most infectious diseases, from gonorrhea to tuberculosis. Most experts assumed that would apply to HIV as well—that treating people with antiretroviral therapy and lowering their HIV viral load would reduce the risk of transmission to others—but definitive evidence has been lacking.

Last year, a landmark National Institutes of Health clinical trial finally provided spectacular proof—a 96% reduction in new infections—of the preventive potential of treatment.