Whats the deference between food poisoning and stomach virus?

And how can you tell witch one you have? My husband has been sick for a few days now with fever, chills, being tired, grumpy. now he has diarrhea. He is thinking it's one of the above how can you tell? Food poisoning is from something you eat the day before so i don't think it's that.He hates the doctors so any ideas? I think it might be from a infection from a tooth but he has no pain, it's way past the nerve and beond. but i ould be wrong..