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Covid-19 coronavirus particle, illustration. The new coronavirus SARS-CoV-2 emerged in Wuhan, China, in December 2019. The virus causes a mild respiratory illness (Covid-19) that can develop into pneumonia and be fatal in some cases. The coronaviruses take their name from their crown (corona) of surface proteins, which are used to attach and penetrate their host cells. Once inside the cells, the particles use the cells' machinery to make more copies of the virus. (KEYSTONE/SCIENCE PHOTO LIBRARY/JOHN BAVARO FINE ART/SCIENCE PHOTO LIBRARY)