No, I DON'T think that is the biggest cause of food poisoning, but it IS A RULE FOR THE WORKERS. And some, if not most, food-borne illnesses come from the hands if they are not washed properly or at all.
I've been in the men's room to relive myself, and I've seen workers come in there and literally walk right back out after using the urinal or toilet and never wash their hands.
When I was in a culinary arts training program, part of the training had included the Serve-Safe study and test, which I've successfully passed, along with the cooking class. And out of all of the possibilities of bacteria, germs & pathogens being spread, washing of the hands, personal hygiene, hairnets and with men, clean shaven faces ARE the #1 priorities to eliminate them in the workplace, especially if you work in a hospital kitchen or a flight kitchen.
Food temps are a big thing as well, be it on the steam table, in the fried or freezer and during food preparations.