A high percentage of the more than 2 million inmates in U.S. jails and prisons suffer from mental illness, addiction or infectious and chronic diseases like HIV/AIDS and diabetes. About a quarter suffer from major depression and a fifth from psychosis. Many had little or no health care before being incarcerated. Providing treatment and preventive care for prisoners who eventually return to society can help stem the spread of infectious disease in communities and keep those with mental illness and addiction from landing back in jail, say public-health officials. While prisoners are, ironically, the only Americans who have a constitutionally guaranteed right to health care, most prison health systems are underfunded and understaffed, making the care they provide spotty at best. Meanwhile, strict sentencing guidelines and three-strikes-and-you're-out laws have created a burgeoning — and aging — prisoner population, which is driving skyrocketing health-care costs even higher.