The biggest cause of foodborne illness is:
employee snacking
poor personal hygiene
a virus
toothpicks

For this problem, taking into consideration the options presented with it, the most probable and the most likely answer to this would be the third one which is a virus.

Well, viruses and bacteria and other pathogens would cause foodborne illnesses. That would be common sense. The thing is, in the presented options, poor personal hygiene is a very strong option that could overthrow the third one. Poor personal hygiene doesn't only speak to the person's under care of his own body, but also speaks to how food is handled. Overall, a virus would be the biggest cause for this. If the food that is stricken with a virus is eaten by a person with exemplary hygiene, then that person would become ill, the same as a person that has poor personal hygiene.