Most bouts of food poisoning clear up on their own. But severe symptoms—like diarrhea that’s bloody or lasts more than three days, vomiting too much to keep liquids down, a fever over 102ºF, or signs of dehydration (like dizziness or infrequent urination)—require medical help. (If in doubt, call your doctor.) Here’s when and how contaminated food typically makes people sick.