Norovirus GI & GII, or Norwalk virus, is the most common cause of non-bacterial gastroenteritis in the world. It is widely known for causing the stomach flu on cruise ships. Three genotypes of this diverse virus, GI, GII, and GIV, can infect humans. Genotype group II, genotype 4 (GII.4) is the most common and accounts for the majority of outbreaks around the world. Norovirus, which can have a sudden or gradual onset, typically develops 24-48 hours after contact with an infected person or ingestion of contaminated food or water.
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