Bacteria, parasites, toxins, or viruses can cause gastrointestinal tract irritation. Other causes may also affect the stomacha and cause stomach flu.

Viruses: Norovirus(in adults) and Rotavirus (in children) are the most common causes of stomach flu. 

although rare, bacteriaand parasites can also cause stomach flu.

Other causes:

• Heavy metals in drinking water
• Toxins in food
• Eating a lot of acidic foods
• certain medications