Like others have said, be extra friendly nd refuse politely on health grounds. Be open if they ask exactly what (if you are comfortable with it). Make up for the fact that they might feel offended in other ways, eye contact, smiling etc or an offer to buy a drink or whatever is suitable in that circumstance.
If you know people that can take a joke, just say 'No thanks, you probably touch yourself with that hand...' and grin
Good luck dude.