What is the nastiest thing done by otherwise reasonable people? They spit on everyone else’s food.
I don’t mean literally ejecting a wad of spit. It’s more insidious.
I used to attend a church that had an annual ice cream supper. This was great—as long as you were in front of a particular lady. She used her fingers to pull ice cream off the spoon, then she slurped her fingers between scooping each flavor. Now that I think of it, she did this with almost any wet food, not just ice cream. Yuck!
People who should know better spit on your food all the time. They lick fingers while cooking or serving, they put utensils in their mouth and then use them on community foods (such as using a personal spoon to scoop ice cream or repeatedly sampling foods with the same utensil while cooking), they redip items into queso or salsa, they place half eaten foods back into shared food sources, they cycle between scooping icing off an uncut cake with a finger then licking said finger, etc. Gross!
For the sake of courtesy and disease prevention (all sorts of diseases are transmitted through saliva), please keep your spit to yourself. Wash your hands if they have been in your mouth, and only use clean utensils on shared food.