You should go home. You're not doing anyone any favours by staying. If your colleagues catch what you have you'll be as popular as chicken pox. Tell your boss that you feel terrible and that you're going home in case you pass it on to anyone else.
Well it doesn't matter if he believes you or not. Just tell him that you don't feel well and you're going home. If you ASK him if you can go home, it will give him an opportunity to say no. If you tell him, there's nothing he can do.