The common causes of headache, diarrhea and weakness are food poisoning and gastrointestinal flu. Food that has been spoiled contains a large number of bacteria, which will produce toxins after entering the digestive system and will enter the blood circulation with the absorption of the body, showing the symptoms of food poisoning, the main symptoms are headache, diarrhea and weakness. Patients should go to the hospital for routine blood and stool tests and use levofloxacin tablets and montelukast for improvement. Gastrointestinal flu is a disease caused by a viral infection of the intestinal system, which leads to an imbalance in the flora of the intestinal system and therefore diarrhea. Since it is caused by a viral infection, the patient will also have the symptoms of a cold, which may manifest as headache and weakness, and needs to be treated with antiviral drugs.