Horse-Itus

Horse-Itus is a deadly disease first seen in the episode "Ryan", although "Rise of the Horseman" actually revolves around Will having this disease. It is a man-made disease made by a man named Ryan. Ironically, we are introduced to the Plumber's Ryan.

The disease is contagious and infects anyone who touches another person (Or creature) with Horse-Itus. In the episode "Rise of the Horseman", Will becomes infected with this disease, causing him to think like a horse, have a horse head, and talk in a Western accent, as well as have a lower IQ than normal, and even possibly cause the victim to become delusional.

For minor cases, it's cure is a grass shake, but for more serious cases, you would need to replace grass with hay.