Hansel Tookes, age 30, is an expert at identifying the detritus of injection drug use. As a public health student on hiatus from medical school, he led a group of researchers who walked the streets of Miami for four months, methodically counting discarded syringes in neighborhoods with high rates of drug arrests. As they crisscrossed more than 800 city blocks, the team spotted 328 used syringes, in parks, lots, and along sidewalks.