Toad disease is, in SuperMarioGlitchy4's videos, a disease which cause the person who get it to sprout Toad's afro. This disease was shown in the blooper "War of the Fat Italians 2014 (200k Special!)". In that video, when Mario took the remote from Nintendofan997 messing with it and SuperMarioGlitchy4 snatch it from him and pressed it. The controller then selected the challenge "The Toad Survival Island". Mario and SMG4 then got both teleported to an island fill of Toad, who gave SMG4 the disease, making him freak out. However, Mario didn't contract the disease, because he got taken away earlier by one of the Toad. It also made an appearance on SM64: War of the Fat Italians 2015 (600k Special) when an patient had an sympton of Toad disease, Mario as the doctor, pull his head off with an crowbar, leaving the Patient headless.
This disease can be caugh by someone who is near a bunch of Toad.
The only effect that happen is that a person sprouts a Toad afro.