Dean Carpenter zigzags his way through a row of men seated in hard plastic chairs at Detroit's Tumaini Center, a crisis support organization for the chronically homeless in Michigan's biggest city. The center has no beds, so some men have been living in those chairs for weeks, even years, while case workers try to secure them housing. Carpenter, the center's nurse practitioner, has seen patients with many ailments over the years: scabies, trench foot and, most recently, hepatitis A, which he is on a mission to vanquish. “You want a hepatitis A vaccine? There's an outbreak in Detroit,” Carpenter says quietly to one older man. The man nods, rises and follows Carpenter to the conference room, where a second nurse practitioner and a team of Michigan State University medical students wait with needles.