It is 1979, and a young man lying on the ground shivering from septic shock is taken to an emergency room, where doctors discover a rare microbe previously assumed harmless. In the ensuing months, the same disease reappears in other victims, all from the Mission District of San Francisco. The epidemic explodes out of control, taking the lives of countless young men, and overwhelming University Hospital microbiologist Lynn Lucas and her colleagues.

Fear grips the city and accusations replace reason, while Lynn and other scientists attempt to determine the source of the lethal outbreak....