How do you know if a stomach upset is food poisoning or a viral bug?

Timing is the most useful clue. Food poisoning from a preformed toxin often hits sharply within one to six hours, is dominated by vomiting, and typically settles within a day. A viral gastroenteritis usually starts more gradually a day or two after exposure, lasts two to three days, and more often involves others in the household in a staggered pattern. Bacterial infection tends to come later still and can bring fever and blood in stools. In practice the management is similar — fluids and rest — but blood, high fever, or symptoms beyond three days need review.