Supernatural (TV series)/Recap/S02/E09 Croatoan

Season 2, Episode 9:

Croatoan
Sam and Dean follow-up on a premonition of Dean killing a defenseless man. They discover that a small town is being torn apart by Croatoan, a demonic virus that turns people into homicidal maniacs. The brothers attempt to defend themselves and the last of the survivors, but just as it appears that Sam has been infected, all the victims suddenly disappear. A blood test reveals that Sam isn't infected and they leave town. In the tag, one of the survivors is revealed to be a demon: he contacts his master using the Goblet of Blood and reports that Sam is immune to the virus.

Tropes

 * Ax Crazy: Everyone with the disease.