NCBI, 2012 viral gastroenteritis NCBI, 2012 viral gastroenteritis http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0001298/ viral gastroenteritis NIH NCBI, 2012 Norwalk virus NCBI, 2012 NCBI, 2012 http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0001298/ stomach flu specified by NIH http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0001298