I've worked from McDonalds to upper class restaurants and can answer this.
Fast food type joints - good neighborhood you are looking at possible sweat in your food during very busy times; bad neighborhood it could be spit to things I don't want to discuss.
Regular restaurants like Friday's, sweat is possible during busy times, but also a chance to get blood, especially if you eat during busy lunches (often lunch cooks do prep early (chopping, slicing, etc)), and I've nearly cut off a finger once and then cooked for lunch (wrapped in a shit-ton of bandages, but still).
High end restaurants are where you'll most likely get sweat, and maybe even something that was dropped on the ground. With all the fancy plate setups, somebody's hands are all over your food, and many times more than one person. They don't tend to look at dropped food like cheaper restaurants might; it just fell, no big deal.
The worst is chow trucks though. Only did this for a short time, and oh the nasty'ness of it all. I'd rather starve than buy something from a food truck.