Hand washing?

I am concerned about my baby getting sick (cold or flu) because of being handled by people who have not washed their hands. I understand that kids need to be exposed to germs in order to build immunity, and I'll be much more willing to let this happen when she is a few months older, but at this time, I want all visitors to our home to wash their hands before holding her. Would it be appropriate to put a sign on the front door indicating this so that I don't need to have the same conversation over and over again? Any other suggestions?