3) Do not care at all, and consider that our bodies are pretty well tuned to handle such thing as a few particles of foreign excrement (or other dirty things).
Seriously, we have become irrational animals afraid of everything. (I would wash my hand after splashing a fly if there was a rotten corpse around, though, and would have done even without the fly because my nose would have alerted me of a chemical danger.)
2) see the fly walk over your food, your face, your kids food...
Do you want to touch a "dirty" fly, or eat what the fly has walked on?
I'll take #1 any day.