When the town of Walkerton, Ontario, is hit by a mysterious illness, hundreds are stricken by vomiting and diarrhea. When the public health investigation finally uncovers the the truth, their findings shock the nation.

When the congregation of the Lutheran church inundates local doctors, they are baffled as to the cause of this outbreak of stomach problems. When one of the patients dies, doctors must solve the mystery before more lives are lost.

When 22-year-old Shelley Cushing collapses while skiing, doctors find nothing wrong with her and suspect a "complicated migraine." Suddenly she is fighting for her life as doctors struggle to figure out what's happened to this seemingly healthy woman.

A couple visiting New York comes down with what they think is influenza, but when they seek medical help, the pair gives doctors the shock of their careers. Their mystery illness turns out to be something much more astonishing and dangerous.

Eight-year-old Joshua Wooten is a real mystery. When his mom takes him to the ER after finding him collapsed on the floor, doctors discover his blood oxygen level is just half of what it should be, and yet he shows no obvious signs of disease.

When 23-year-old Sebastian Hitzig is found collapsed with blood oozing from his nose and ears, doctors are faced with a number of deadly possibilities. But as they attempt to narrow these down, Sebastian begins to rapidly deteriorate.