Adenovirus occurs mainly in children under the age of 2 years. Of the 49 types of adenoviruses, one strain affects the gastrointestinal tract, causing vomiting and diarrhea. Symptoms typically appear 1 week after exposure. Adenovirus infections occur year-round.

 

Caliciviruses cause infection in people of all ages. This family of viruses is divided into four types, the noroviruses being the most common and most responsible for infecting people. The noroviruses are usually responsible for epidemics of stomach flu, and occur more frequently from October to April. Infected people experience vomiting and diarrhea, fatigue, headache, and sometimes muscle aches. Norovirus symptoms appear within 1 to 3 days of exposure. It is now thought that at least 50 percent of all foodborne outbreaks of gastroenteritis can be attributed to noroviruses. Norwalk virus is a type of norovirus.

 

Astrovirus infects primarily infants, young children, and the elderly. This type of stomach flu virus is most active during the winter months. Vomiting and diarrhea appear within 1 to 3 days of exposure.