In November, 2010, the New England Journal of Medicine published the results of a three-year clinical trial, funded by the National Institutes of Health, announcing the arrival of a treatment that could reduce the risk of contracting H.I.V. by more than ninety per cent. But, in fact, adoption of the drug has been slow. According to Dawn Smith, a biomedical interventions implementation officer in the C.D.C.’s epidemiology branch, at least half a million Americans are good candidates for PrEP—meaning that they are at high risk for contracting H.I.V.