An awful disease that causes the personaffected by it to have trouble reading and often inhibits their ability to learn.
Dylan: Wow, man, did you hear Jeff has diphtheria?
Joey: Shit, that's tough. How's he gonna get through school?
Dylan: I don't know, man.
Dani: Let's just all be thankful he doesn't have dyslexia.
Diptheoryia is an illness dat is often experienced by an individual who is himself of very limited moral and/or intellectual standing, lacks basic motivation or common sense, etc., but who fancies himself superior to many others; arrogant rich-bug types like Tronald Dump come to mind.