A highly contagious fungi, originating on the buttocks, it reproduces asexually, using air-born spores, which when air-borne can contaminate others who come in contact with a carrier. When the spores germinate, they expand and consume whatever food source is on hand, be it living or dead.
What is most disturbing is if a female living carrier reproduces, it infects the new born child making an almost unstoppable highly contagious super being, known as an ass baby. (refer to Ass Baby)
A cure for Ass Fungi is yet to be found.