Once at work we were told that it has been prooven that traces of urine can be found in the bowl filled with nuts placed on counters in bars. People go to the toilet don't wash their hands, or don't do it properly, and then dig their dirty hands into the nut bowl! I haven't touched nuts/crisps since unless they've come straight from a packet.
That reminds me of a woman I used to work with. She hated double-dipping. Don't know what double-dipping is? Imagine you're at a social function, catered with finger food. You've been enjoying some of the dipping sauces on offer. And then you notice someone dipping a cocktail sausage in the dipping sauce, taking a big sloppy bite of the sausage and then re-dipping the remaining, half-eaten piece of sausage, drool and all, back into the dipping sauce. Yetch! Anyway, this woman I worked with had such a aversion to double-dipping, she was known to yell at an offender, from across a crowded room, "No double-dipping!!!" And I, for one, was glad she did.