Gastroenteritis

Gastroenteritis is a general term for any infection that results in inflammation of both the stomach and small intestine. It can be characterized by a mix of symptoms that will include one or more of diarrhea, nausea, vomiting, and abdominal pain. In children (in whom the disease is far more common), it's most often caused by rotavirus, but in adults, it's more likely to be caused by norovirus or Campylobacter. It can also be caused, less commonly, by other viruses. Bacteria and parasites can also result in the condition.
Treatment is with fluids. Generally, oral fluids will suffice, but in severe cases, fluids must be given intravenously. If caused by a virus, the condition will usually resolve itself within seven days without any other treatment. However, if the cause is bacterial or from a parasite, further treatment will be required. A key indicator that the cause is not viral is blood in the stool.
Gastroenteritis is very common, with several billion cases per year. It's more common in the developing world. Despite its low mortality rate, it kills over 1 million people a year.
Gastroenteritis at Wikipedia
Viral gastroenteritis at Mayo Clinic