Hell, I've shaken hands with African tribesmen who likely have never seen soap in their lives. I somehow managed to survive. We just kept our hands away from our face until we had an opportunity to wash/purel our hands well. I think in the western world your chances of getting something from a hand shake are pretty slim.
If you're not feeling well, or someone around you isn't then it's courtesy to say "I'm coming down with something, you don't want to shake my hand." You could use that with someone you think might be an unwashed heathen too. Initiate with a "I'm sick so I won't shake your hand." problem solved.
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