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Covid-19 coronavirus. Illustration showing the structure of the SARS-CoV-2 virus particle. At the virus's core is its RNA (ribonucleic acid) genome (coils). Embedded in the viral envelope (grey) are spike proteins (red) that the virus uses to attach to and infect a host cell. This virus emerged in Wuhan, China, in December 2019, and causes a mild respiratory illness (covid-19) that can develop into pneumonia and be fatal in some cases. (KEYSTONE/SCIENCE PHOTO LIBRARY/JOHN BAVARO FINE ART/SCIENCE PHOTO LIBRARY)