To be obsessively and compulsively changinging any font in anything on the internet for no particular reason other than to "Try something different"
Most cases of Fontaculosis results in the infected getting confused with which writing is which, as they regularly forget which font was theirs.
Example for Fontaculosis:
Tom: Lol yeah I thought so. (Crazy rainbow font with Italics and underline)
Bradly: Who the hell is writing this, stop changing your font man, I can't even understand it, and its a bad font. You're not cool, give up you noob
Tom: Oh.. I see.. (Plain black Times New Roman)
Bradly: You fail. Get checked for Fontaculosis.