What are the causes for Gastroenteritis
(Stomach Flu)?

Gastroenteritis has many causes. Viruses and bacteria are the most common.Viruses and bacteria can be contagious and can spread through the consumption of contaminated food or water. In up to 50% of diarrheal outbreaks, no specific agent is found. The infection can spread from person to person because of improper handwashing following a bowel movement or handling a soiled diaper.Gastroenteritis caused by viruses may last one to two days. However, some bacterial cases can continue for months.

Source: http://www.emedicinehealth.com