Gastric flu is a common term used for infectious gastroenteritis, which causes small outbreaks in the workplace, schools and within the home. There are many causes of infectious gastroenteritis – a number of bacterial, viral and protozoal species may cause this inflammation of the stomach, small and large intestine.he term gastric flu specifically is commonly used to describe viral gastroenteritis, along with other terms like intestinal flu, tummy flu and stomach flu.