Indooritis is defined as "the hidden threat in every home, mall, and office that drives sufferers to strange, synthetic extremes to replace the natural joys in life."
So basically it just means that when people spend too much time indoors they go a little nuts.
Do you have it? I think everybody does a little by the time spring rolls around. But now that spring has sprung it's much easier to get outside, and Lipton tea is launching a new campaign to "fight indooritis." They've got a website full of interesting features and strange facts about the outdoors (did you know a koala's fingerprints are so similar to a human's they could be confused at a crime scene?), and they've even come up with "O.U.T.S.I.D.E," which stands for "organization for the understanding and treatment of society's indoor dependency epidemic."
This idea (and the website especially) is lighthearted and silly and I think it's great idea.