Increasing numbers of adolescents in Lyme, Connecticut, are being diagnosed with rheumatoid arthritis, Bell's palsy, or the flu, and it seems that half the town is hobbling around on crutches. Philadelphia veterans develop high fevers and die within days. Rashes and sores break out on the poor in the South; some go insane. From New York and Connecticut to Michigan and the South, silent epidemics are killing Americans. What causes these strange diseases? Follow the epidemiologists—disease trackers and detectives—as they uncover the answers to six mystifying cases.