Gastroenteritis is a condition that causes irritation and inflammation of the stomach and intestines. Diarrhea, crampy abdominal pain, nausea, vomiting, headache, and fever are the most common symptoms. Viral infection is the most common cause of gastroenteritis but bacteria, parasites, and food-borne illness (such as shellfish) can be the offending agent. Many people who experience the vomiting and diarrhea that develop from these types of infections or irritations think they have "food poisoning," and they may indeed have a food-borne illness. Many people also refer to gastroenteritis as "stomach flu," although influenza has nothing to do with the condition. 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 you vomit and experience diarrhea. Most people recover easily from a short bout with vomiting and diarrhea by drinking fluids and easing back into 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.