Gastroenteritis is a condition that causes irritation and inflammation of the stomach and intestines (the gastrointestinal tract). The most common symptoms are
  • diarrhea, 
  • crampy abdominal pain, 
  • nausea, 
  • vomiting.
Many people also refer to gastroenteritis as "stomach flu." This can sometimes be confusing because influenza (flu) symptoms include headache, muscle aches and pains, and respiratory symptoms, but influenza does not involve the gastrointestinal tract.
The term stomach flu presumes a viral infection, even though there may be other causes of infection.

Viral infections are the most common cause of gastroenteritis but bacteria, parasites, and food-borne illnesses (such as shellfish) can also be the offending agents. Many people who experience vomiting and diarrhea that develops from these types of infections or irritations think they have "food poisoning," when they actually may have a food-borne illness.

Travelers to foreign countries may experience "traveler's diarrhea" from contaminated food and unclean water.

The severity of infectious gastroenteritis depends on the immune system's ability to resist the infection. Electrolytes (these include essential elements of sodium and potassium) may be lost as the affected individual vomits and experiences diarrhea.

Most people recover easily from a short episode of vomiting and diarrhea by drinking fluids and gradually progressing to a normal diet. But for others, such as infants and the elderly, loss of bodily fluid with gastroenteritis can cause dehydration, which is a life-threatening illness unless the condition is treated and fluids restored.

Resource: www.emedicinehealth.com