A disease which completely takes over the lives of the victims by making them lurk on 4chan for hours on end everyday. It is usually contracted once an otherwise intelligent and responsible human being is called a newfag too many times, gets butthurt and then decides to look up 4chan lingo on urban dictionary in an effort to try and make himself look like an oldfag on the message boards. Sadly, they are usually found out, get butthurt again and leave /b/ crying, only to return again tomorrow.
Greg: Have you seen Tom lately? He looks like s**t and he's always muttering about something called /b/.
Jerry: Yeah man, he's got 4chanitis.
Alec: What a newfag.