Wilmington, Del., has a big problem: Large groups of black people are going crazy. And this collective “mental illness” is causing record levels of crime and gun violence in this mostly black town of 70,000. That is the official diagnosis of the city council, which, by unanimous agreement earlier this month, asked the Centers for Disease Control to investigate a wave of psychological mayhem that has turned this historic and once-charming city into an unrecognizable husk of its former self. Chief diagnostician of this crisis in public health is city council member Hanifa G.N. Shabazz: “There is a well known...