GII.4 Sydney is a strain of Norovirus first discovered in March 2012 and is now the predominant strain of recent norovirus outbreaks worldwide, replacing G11.4 New Orleans. It is responsible for millions of cases in Australia, France, New Zealand, Japan, and elsewhere. England and Wales reported 63% more cases for this strain than for last year's strain. It belongs to Genogroup II, genotype 4, or GII.4.
The virus can live on hard surfaces for several weeks and as few as 18 viral particles (virons) are enough to make a person sick (compared to 1,000 for the common flu). Common home cleaning agents do not kill the virus and it is only removed from a person's hands by repeatedly washing them.
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Norovirus GII.4 Sydney Wikipedia(Text) CC BY-SA