A psychological disorder. Usually occurs when a person pledges a fraternity and suddenly thinks they're hot shit, despite being, in fact, the same exact person as they were before bid day. Symptoms include sleeping in khakis; being rude, arrogant, standoffish, aloof, or dismissive toward GDIs; and belligerent/excessive swagger.
Pledgitis usually peaks during the pledging season and recedes with initiation, when the novelty of being a brother begins to wear off. At this point, the person in question will usually revert to his or her normal self.
How a pledge deals with his pledgitis will decide whether or not he will become a true fraternity man or a simple frat boy. Whether or not a fraternity sees pledgitis as a temporary phenomenon or as a permanent staple has a large part to play in its ethos.
Jon: "Hey Mike, I'm about to head out to dinner. Wanna join me?"
Mike: "Sorry bro, I'm eating with the frat tonight."
Jon: "Dude, I haven't seen you in five weeks."
Mike: "Don't be gay, I'm not your boyfriend."
Jon: "Mike, you better get over your pledgitis before someone shoots you in the face."