Gastroenteritis is most often caused by infections of the stomach and intestines, usually from viruses, but also from bacteria, parasites, and sometimes toxins, chemicals, or medications. It typically spreads through contaminated food or water, unwashed hands, and close contact with someone who is already sick.

Main medical causes

  • Viral infections are the most common cause (often called ā€œstomach fluā€), especially norovirus and rotavirus. These spread easily in households, schools, cruise ships, and aged-care facilities.
  • Bacterial infections such as Salmonella, Campylobacter, Shigella, and certain strains of E. coli can cause gastroenteritis, often linked to undercooked meat, poultry, eggs, unpasteurized milk, or contaminated water.
  • Parasitic infections like Giardia and Cryptosporidium are less common but can occur after drinking contaminated water or swimming in contaminated pools.
  • Toxins and nonā€‘infectious triggers include bacterial toxins in food (food poisoning), heavy metals in water, some seafood toxins, acidic foods, and some medicines (for example, antibiotics or chemotherapy), which can all irritate or inflame the gut.

How it usually spreads

  • Eating food that has been left at room temperature too long, not cooked properly, or prepared in unclean conditions.
  • Drinking water contaminated with sewage, animal waste, or industrial pollution, which is a particular issue in some regions and after floods or natural disasters.
  • Close contact with an infected person, sharing utensils, or touching contaminated surfaces then touching the mouth without washing hands.

Everyday risk factors

  • Not washing hands properly after using the toilet, changing nappies, or handling raw meat.
  • Eating outside food from unhygienic vendors or buffets where food is kept warm for long periods.
  • Traveling to areas with poor sanitation (ā€œtravelerā€™s diarrheaā€), especially if drinking tap water or eating raw foods.
  • Weakened immune system, very young children, and older adults are more likely to get gastroenteritis and to become more seriously ill.

When to worry

  • Signs like persistent vomiting, blood in stool, very high fever, severe stomach pain, or signs of dehydration (little urine, dry mouth, dizziness) can mean a more serious cause of gastroenteritis and need urgent medical review.
  • Symptoms that last more than a few days, especially with weight loss or in young children or older adults, should also be checked by a healthcare professional.

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