Ghost sickness is spread by a specific class of spirit known as the Buruburu. First the victim must have physical contact with the ghost's body or the body of another victim of ghost sickness. However, not everyone is susceptible to the virus. The only people who can contract it are those that share a specific personality that matches the original killer of the Buruburu. In this case, the killers of Luther Garland used fear as a weapon.
Ghost sickness first appears as nightmares and subtle signs of irrational fear. The fear gets greater and greater as the illness progresses. Soon after the initial symptoms, wounds mirroring those the originating ghost received while alive appear on the victim without an apparent cause. As the ghost sickness progresses further the victim feels queasiness that degenerates into vomit. Toward the ending stages the vomiting degenerates further into vomiting blood. At the final climax of the illness the victim will experience intense terror that will cause a fatal heart attack.
The cure for ghost sickness is to destroy the ghost who began spreading the virus. However, as Luther Garland's body had been spread across the road, precluding the routine salt and burn, Bobby suggested the Buruburu could be "scared to death" by recreating its original death, and employed the use of an iron chain etched with spellwork to bind it.
Sam and Dean are investigating a series of deaths, when Dean starts to act strangely. They call Bobby, who diagnoses the problem as ghost sickness.  As Sam and Bobby race to find a means to save Dean, Dean suffers from the symptoms, which include itchiness on his arms, fear about everything, and hallucinations of Azazel and Lilith.