A virus is the most common cause of gastroenteritis. Rotavirus is the most common virus causing gastroenteritis in children. Adenoviruses are another common group of viruses that cause gastroenteritis in children.
Viruses are easily spread from one infected person to another by close contact. This is often because the virus is present in the infected person's hands after they have been to the toilet. Surfaces or objects touched by the infected person can also allow transmission of the virus. The virus can also be passed on if the infected person prepares food. Outbreaks of a virus causing gastroenteritis can often occur - for example, in schools or hospitals.
Food poisoning (from eating food infected with microbes) causes some cases of gastroenteritis.