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The End of the GII.4 Norovirus Era? On July 2, Eurosurveillance published two articles on the unusual prevalence in Japan of a previously rare norovirus genotype (GII.17 Kawasaki 2014)1,2. The norovirus genus comprises seven genogroups (G) subdivided into 30 genotypes. Although viruses from genogroups GI, GII, and GIV can infect humans, GII.4 viruses have caused the majority (70–80%) of norovirus-associated acute gastroenteritis (AGE) outbreaks since the mid-1990s. In the future, GII.4 viruses will likely continue to cause a large number of AGE cases, as they mutate easily, allowing them to evade immunity acquired by their hosts during previous exposures. The first GII.17 virus strain was identified in 1978. Since then, this strain has been detected sporadically on several continents (Africa, Asia, Europe, North and South America). It has been associated with AGE in children and adults and with chronic infections in a kidney transplant recipient and a leukemia patient. During the winter of 2014–2015, it caused AGE outbreaks in Guangdong Province, China (24 out of 29 outbreaks), leading to an increase in the number of outbreaks and cases: 29 outbreaks comprising 2,340 cases, compared to 9 outbreaks and 949 cases the previous winter, when the GII.4 strain was the dominant genotype. An increase in outbreak activity can also be attributed to this new virus during the same winter in Jiangsu Province, China. It was also dominant in Hong Kong and Taiwan, as well as in Japan. Sporadic cases have been detected abroad, notably in France in early 2013. New strains of norovirus can spread rapidly around the world. So far, GII.17 has replaced the GII.4 strain only in Asia. However, given the precedents in China and Japan, increased epidemic activity would likely result from the replacement of GII.4 by GII.17 on other continents. Nevertheless, the spread of a new strain is not inevitable, as demonstrated by the precedent of the GII.4 Asia 2003 strain. The currently dominant GII.4 Sydney strain may not be replaced in other regions of the world; this remains to be seen: public health professionals and surveillance systems must nevertheless prepare for a possible increase in norovirus epidemic activity caused by this new genotype.