The Centers for Disease Control and Prevention (CDC) estimates that noroviruses cause nearly 21 million cases of acute gastroenteritis annually, making noroviruses the leading cause of gastroenteritis in adults in the United States. Of the viruses, only the common cold is reported more often than a norovirus infection—also referred to as viral gastroenteritis.
Nature has created an ingenious bug in norovirus. The round blue ball structure of norovirus is a protein surrounding the virus’s genetic material. The virus attaches to the outside of cells lining the intestine, and then transfers its genetic material into those cells. Once the genetic material has been transferred, norovirus reproduces, finally killing the human cells and releasing new copies of itself that attach to more cells of the intestine’s lining.
Norovirus (previously called “Norwalk-like virus” or NLV) is a member of the family Caliciviridae. The name derives from the Latin for chalice—calyx—meaning cup-like and refers to the indentations of the virus surface.  The family of Caliciviridae consists of several distinct groups of viruses that were first named after the places where outbreaks occurred. The first of these outbreaks occurred in 1968 among schoolchildren in Norwalk, Ohio. The prototype strain was identified four years later, in 1972, and was the first virus identified that specifically caused gastroenteritis in humans. Other discoveries followed, with each strain name based on the location of its discovery—e.g., Montgomery County, Snow Mountain, Mexico, Hawaii, Parmatta, Taunton, and Toronto viruses. A study published in 1977 found that the Toronto virus was the second most common cause of gastroenteritis in children. Eventually this confusing nomenclature was resolved, first in favor of calling each of the strains a Norwalk-like virus, and then simply, a norovirus – the term used today.
Humans are the only host of norovirus, and norovirus has several mechanisms that allow it to spread quickly and easily. Norovirus infects humans in a pathway like the influenza virus’ mode of infection. In addition to their similar infective pathways, norovirus and influenza also evolve to avoid the immune system in a similar way.