There is no "stomach virus" per se. Viruses that cause the condition most commonly known as "stomach flu" actually affect the small intestine. Symptoms of these infections include vomiting, diarrhea, and sometimes body aches. Anyone can become infected, although infants and children, the elderly, and people with weakened immune systems may have a more severe form of the infection.
Stomach flu is caused by a virus. There are a number of different viruses that can cause stomach flu, but the so-called "stomach virus" does not actually affect the stomach at all. Viruses that cause stomach flu affect the small intestine. Furthermore, these stomach flu viruses are not actually flu viruses. The flu virus is the influenza virus. Influenza virus does not cause stomach flu symptoms.
Regardless of the inaccuracies of the terms "stomach flu" and "stomach virus," these are the most commonly used terms to describe what healthcare providers call viral gastroenteritis.
There are a number of viruses that can cause stomach flu. There are four main types of stomach viruses:
Rotavirus is the leading cause of stomach flu among children 3 months to 15 months old and the most common cause of diarrhea in children under the age of 5 years. This type of "stomach virus" causes vomiting and watery diarrhea for 3 to 8 days, along with fever and abdominal pain. This stomach flu virus can also infect adults who are in close contact with infected children, but the symptoms in adults are milder (see Adult Rotavirus). In the United States, rotavirus infections are most common from November to April.