People mainly get diarrhea from food contaminated due to unsafe farming, prep, or cooking practices. Fast food restaurants may be particular prone to this due to underpaid staff and buying produce and meat from the lowest bidder.

When a supplier is at fault, it can cause mutli-city outbreaks which attracts the attention of investigators. They have many times identified the souce of the contamination and the organism responsible.

That better describes what is happening in this case than “fast food is bad for you.”