Croatoan Virus

The Croatoan Virus is a supernatural virus that infects people and turn them into murderous zombie-like creatures. It is transmitted through blood to blood contact. However Sam Winchester is shown to be immune to its effect. Lucifer and Pestilence hoped to use this virus to wreak havoc on Earth during the Apocalypse, but it was averted by Sam and Bobby.

Croatoan
The Winchester brothers arrived at River Grove, Oregon, following a vision he had of Dean killing a man. As the entire town began to turn, Sam and Dean retreated to a local clinic where a doctor was studying the blood of an infected person and found traces of sulfur in the blood, which lead to the brothers concluding that a demon was at work.

The virus is spread by blood to blood contact and usually takes three to four hours for the symptoms and sulfur to manifest. However, Sam, who had had blood contact with one of the infected persons, showed none of the symptoms, even after five hours following his infection. When the doctor ran tests to check his blood, he found no traces of sulfur at all.

Near the end, it is revealed that the one responsible behind this virus is Duanne Tanner, the man whom Sam saw Dean kill and who was in fact possessed. Communicating with Azazel by the means of the Goblet of Blood, he informed the Yellow-Eyed Demon that "The Winchester boy [Sam] is immune".

The End
In the alternate future, the virus had been released in 2012 by Lucifer and had infected many people around the world. Those who were infected, called "croats", were shot on sight. Following an outbreak in Dallas, President Pallin ordered its bombing.

Future Dean also shot one of his men, who was infected, without warning.