Stomach flu is usually caused by a viral infection of the stomach and intestines, most often from norovirus or rotavirus , picked up from contaminated food, water, or close contact with someone sick.

What the “stomach flu” really is

  • “Stomach flu” is the common name for viral gastroenteritis , an infection of your stomach and intestines, not the same as influenza (the real flu, which is a lung infection).
  • It leads to inflammation of the gut lining, which causes diarrhea, vomiting, cramps, and nausea.

Main causes

  1. Viruses (most common)
    • Norovirus : Top cause in adults and a leading cause of foodborne illness worldwide.
 * **Rotavirus** : Very common cause in young children, especially before vaccination; spreads easily in daycare and families.

 * Other viruses include astrovirus and adenovirus, which can also infect the gut.
  1. How these viruses spread
    • Eating or drinking contaminated food or water (for example, food handled by someone infected).
 * **Close contact** with a sick person: sharing utensils, being in crowded or enclosed spaces, caring for someone with vomiting/diarrhea.

 * Touching **contaminated surfaces** (bathroom fixtures, doorknobs, phones) and then touching your mouth.
  1. Other germs that can cause similar “stomach flu” symptoms
    • Bacteria (like Campylobacter , Salmonella , certain E. coli , Shigella) often from undercooked meat, unpasteurised milk, or unsafe water; these are usually called food poisoning but feel similar.
 * **Parasites** (like _Giardia_ , _Cryptosporidium_) from contaminated water, food, or animal contact.

Why it hits some people harder

  • Infants, older adults, and people with weak immune systems are at higher risk of severe dehydration and complications.
  • In most healthy adults, viral stomach flu is miserable but usually short-lived and self-limiting.

Quick practical example

  • Someone with norovirus vomits in a shared bathroom. Tiny viral particles contaminate the sink, tap, and door handle.
  • Another person touches the handle, doesn’t wash hands well, eats a snack with those hands, and then develops vomiting and diarrhea a day or two later.

TL;DR: Stomach flu is mainly caused by viruses (especially norovirus and rotavirus) that infect the gut and spread through contaminated food, water, surfaces, or close contact with someone who’s sick.

Information gathered from public forums or data available on the internet and portrayed here.