A molecular surface model of the Norwalk virus, a species of Norovirus responsible for viral gastroenteritis. The virus is usually transmitted through physical contact or contaminated food and water, and can be extremely contagious. Once the host is infected, the virus replicates in the small intestine producing symptoms such as nausea, vomiting, diarrhea, abdominal pain, lethargy and muscle aches. Viral infection is more common during the winter when people spend more time indoors and in close proximity to one another.