Illnesses which are spread around at science fiction and furry conventions due to the large numbers of people who gather and interact in one confined place. Lack of sleep, drama, stress, travel, and post con depression may also depress the immune system, contributing to con crud.
A particularly nasty, crusty, fast advancing rash or skin eruption. For some reason it appears to strike only chrome-plated, 360 degree assholes, while leaving the virtuous unscathed. By so doing, it restores ones faith in the mysteries of life.
Alarmed Fellow:"Don't sit in that chair---Mayson just got up from there, and he has the galloping crud!"
Unconcerned Grand Guy:"Don't worry--Mayson's a dirt-bag, but I have my aura of righteousness to protect me."