Asperger's Syndrome, sometimes usually referred to as just Asperger's, is a developmental disorder on the mild end autism spectrum that is characterized by significant difficulties in social interaction and possible lack of communication, as well as restrictive and repetitive behaviors and interests.
In the Season Fifteen episode "Ass Burgers", Cartman mistakenly believed the disorder to involve literal "ass burgers", to which he put homemade hamburgers in his underwear. When found out by the nurse, Cartman ended up selling these burgers, with his "special ingredient" to other students.
Stan Marsh is erroneously diagnosed as having Asperger's Syndrome, due to his cynicism and lack of motivation.
His cynicism is thought to have been brought on by him having been vaccinated for the flu, a practice that some believe to cause autism and other neurological problems in children, a claim which has been widely disregarded by the scientific community.