GASTROENTERITIS CAUSES

Most cases are caused by viruses, but not by the same viruses that cause influenza. A leading culprit is rotavirus, a common germ that is easily spread through physical contact, such as shaking hands or sharing eating utensils. Rotavirus is especially likely to show up in daycare centers.

About one in five people with "stomach flu" has a bacterial infection, often acquired through food poisoning or drinking untreated water. Common bacterial sources of the disease include Salmonella, E. coli, and Campylobacter.

Other possible causes of gastroenteritis include parasites (organisms larger than viruses or bacteria), toxins from tainted shellfish, and reactions to medications (including antibiotics and laxatives). It's also linked to chronic diseases such as ulcerative colitis or Crohn's disease.

Because disease-causing bacteria and viruses thrive in areas with poor sanitation, gastroenteritis often strikes travelers to developing countries. That's why up to half of the people who travel abroad to developing countries come back with a form of gastroenteritis called traveler's diarrhea, according to eMedicine. Domestic travelers should also be wary. Tiny parasites that attack the stomach also live in cold climates in the U.S., and they can be picked up if you drink water while swimming in or camping near rivers and lakes.