Ghost sickness is spread by a specific class of spirit known as the Buruburu. First, one must have physical contact with the ghost's body or the body of a ghost sickness victim. Second, one must share a specific personality trait with the original killer of the Buruburu. As a result, not everyone who comes into contact with a Buruburu or someone with ghost sickness is susceptible to the virus. In the case of Luther Garland, his killers used fear as a weapon, and so Dean is susceptible while Sam is not.
Ghost sickness first manifests 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 any apparent cause. As the ghost sickness progresses further, the victim feels queasiness that degenerates into vomiting. Toward the end 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. Dean does not vomit, but seems to experience intense terror that could lead to a heart attack when he hallucinates that Lilith is in his motel room with him.
The cure for ghost sickness is to destroy the ghost who began spreading the virus. However, as Sam and Dean do not have this option--Luther Garland's body had been spread across a long stretch of road, precluding the routine salt and burn--Bobby suggests the Buruburu can be "scared to death" by recreating its original death. They tie an iron chain around the neck of Luther's spirit and drag him along the same road he was killed on, effectively killing him again and curing Dean of ghost sickness before he can succumb to a fatal heart attack.