Any eating place with employees is vulnerable to despicable actions by those employees.
The key is good management and selectivity in hiring. McDonald's probably rates higher than many other eating establishments. In the Fast Food category, I would trust Chick-fil-A above most other restaurants.
Demographics of the neighborhood and available employee pool matter greatly, especially when very different from that of the individual dining patron. We are blessed to live in a relatively benign area.
I now avoid all fast food restaurants except Chic-fil-A. I do not need the extra seasoning that I may get. Not that it will happen, but that it could happen.
There are some fast food places I now avoid because the servers have very obvious and poor hygiene. If you stop for fast food be sure to visit one where the kitchen is on full view. I like our local In and Out because it is totally open to view.