In last year’s outbreak of HIV and hepatitis C in a rural Indiana county, nearly 200 people contracted HIV because of injection drug use and opioid addiction. To prevent a repeat, the U.S. Centers for Disease Control and Prevention (CDC) has compiled a list of 220 counties across the nation where injection drug users are prone to similar outbreaks. Researchers looked at factors such as unemployment rates, overdose deaths and the sale of prescription painkillers. More than half—56 percent—of the counties are in Kentucky, Tennessee and West Virginia, with groupings in Arizona, Utah, California, Missouri, Michigan and 18 additional states. HIV cases related to injection drug use have decreased a lot in the past three decades—they accounted for about 6 percent of HIV infections in 2014, compared with 30 percent in the mid 1990s. But as John Brooks, the lead author of the CDC analysis, tells The Wall Street Journal, progress like that could easily be lost if we don’t learn our lesson from Indiana.