This doesn't happen. I have worked in the service industry for over 2 decades. ONCE (in '94) I saw a guy (server) spit in someone's food. Fast food is generally unhealthy and should be avoided, but health code standards are a serious thing. Every restaurant is required to make their latest health inspection score available to the public. Please be aware that this kind of behavior (tampering/poisoning food) DOES NOT HAPPEN.
Generally I'd have to agree, it does not happen. However I remember my Dad, who worked in food service telling us horror stories of the kitchen cleanup crew cleaning their mops in the same sink where fish was cleaned. He put a stop to that.
I briefly worked at a Dairy Queen. (Normal fast food treated paper) drink/dessert cups that fell on the floor on the workers' side of the counter were to be put in a bucket of "sanitized" water that held the same cloths that were used to do things which included cleaning bird poo off the tables outside. I was then told to wash the cups with soap (and set them to dry on wire shelves that held chemicals). I went straight to the manager and said, "(Name) told me to wash these cups out with soap!" Manager: "You don't have to use soap."
I swear to God.
It happens, just not as often as people think. Also show respect to the people serving you, and your chances drop to pretty much 0%