Noroviruses were previously called Norwalk or Norwalk-like viruses. They are a group of non-enveloped, single-stranded RNA viruses that cause acute gastroenteritis (inflammation of the stomach or intestines). Noroviruses belong to the family Caliciviridae, which cause acute gastroenteritis.
Noroviruses are named after the original Norwalk strain, which caused an outbreak of gastroenteritis in a school in Norwalk, Ohio in 1968.
Currently, at least 33 different norovirus genotypes have been described. Noroviruses that infect humans are caused by genotypes belonging to three genogroups (GI, GII, and GIV). Variants of the GII.4 genotype have been the most common cause of norovirus illnesses worldwide. In recent years, previously rare genotypes (GII.17, GII.2) have emerged and caused the majority of norovirus illnesses is some parts of the world.
Since there are many different genotypes of noroviruses, you can get infected many times during your life. You can develop immunity to (protection against) norovirus, but only for a short period of time (6 months to 9 years). This may explain why so many people of all ages get infected during norovirus outbreaks. Also, whether you are susceptible to norovirus infection is determined in part by your genes.