The extreme obsession with Sonic the Hedgehog, or any other sonic character. The person may talk about A LOT or may play the games and dis family and friends.
How to tell if someone has Sonicitus-- you would know (its evident)
Cure-- There is no cure (you should just join them)
A person who collects sonic games, makes sonic holidays, collects sonic comics, sonic is the only thing they talk about, and constantly writes sonic on papers and such and will actively admit he/she has Sonicitus.