Common side effects include:
Staying up all night on twitter
Checking twitter on your phone every two seconds
Having an epic freakout when deprived of twitter for more than 10 hours
Failing exams/not doing homework/being late to work because you were on twitter
Person 2: I have tweetitis.
Person 1: Stop being so anti-social and put your freaking phone away!
Person 2: I can't I have tweetitis!
Person 1: *on phone* Uh hi, we've been without internet for about 9 and a half hours... what's happening?
Person on phone: Sir, we'll get to you as soon as we can but you're quite far down on the list so it'll be another couple hours
Person 1: I have tweetitis
Person on phone: We'll be right there! Don't want another freak out on our hands!
Teacher: Why isn't your homework done
Pupil: I have tweetitis
Parent: Why are your exams results so bad?!
Kid: I have tweetitis
Boss: You've been late to work 5 days in a row! You better give me a good reason not to fire your ass!
Employee: I have tweetitis