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Covid-19 coronavirus particles, illustration. Each virus consists of a core of RNA (ribonucleic acid, not seen) genetic material surrounded by a protein nucleocapsid and a lipid envelope (purple). The coronaviruses take their name from their crown (corona) of surface proteins (red and gold), which are used to attach to and penetrate their host cells. Once inside the cells, the particles use the cells' machinery to make more copies of the virus. The new coronavirus SARS-CoV-2 (previously 2019-CoV) 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. (KEYSTONE/SCIENCE PHOTO LIBRARY/DETLEV VAN RAVENSWAAY/SCIENCE PHOTO LIBRARY)