Rotavirus is an extremely common viral illness that causes severe diarrhea in young children. Most children will have a bout of rotavirus illness before the age of 2. Another common cause of vomiting and diarrhea is norovirus. Noroviruses are the cause of those cruise ship outbreaks that have been in the news lately.

http://www.cdc.gov/ncidod/dvrd/revb/gastro/rotavirus.htm