When HIV, the virus that causes AIDS, was first identified in the 1980s, it almost certainly a death sentence. More than 100,000 Americans died from the new disease in that decade. The idea that one day there might be medicines to prevent infection was a fantasy. Today, these medicines, called pre-exposure prophylaxis (PrEP), do exist, but most people who could benefit don’t take them. The policies that govern these medicines — and the enforcement of those policies — are largely to blame. PrEP is wildly effective. PrEP reduces the risk of getting HIV from sex by 99%, and from injection…