Gastroenteritis is a painful illness in which your stomach and your intestines become swollen and angry at the same time. You may vomit, you may have diarrhoea (watery poo), and you usually get cramps in your belly; sometimes there is a little fever too. Doctors and hospital notes use this long word. Everyday people more often say a stomach bug, or, loosely, stomach flu — though actually it is not the flu at all, which is a different virus in your nose and chest. A lot of cases come from a virus you catch from other people; some come from food or water that had bacteria in it, which people then call food poisoning. It is a formal medical noun. It often lasts a few days. The real danger, especially for small children and older people, is losing too much water.