Illustration based on a transmission electron micrograph (TEM) showing the difference in size of a norovirus (blue), Sars-CoV-2 virus (orange-red) and influenza virus (violet). The norovirus has a diameter of 30-40 nanometres (nm), Sars-CoV-2 80-140 nm and the influenza virus 80-120 nm. Norovirus causes about half of all gastroenteritis cases around the world. SARS-CoV-2 emerged in Wuhan, China, in December 2019. The virus causes the disease Covid-19, a respiratory illness that can lead to pneumonia and can be fatal in some cases.