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Illustration of a coronavirus particle. Different strains of coronavirus are responsible for diseases such as the common cold, gastroenteritis and SARS (severe acute respiratory syndrome). 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. The coronaviruses take their name from their crown (corona) of surface spike proteins (large protrusions), which are used to attach and penetrate their host cells. Also within the membrane are haemagglutinin-esterase glycoproteins (smaller protrusions), small envelope glycoproteins (holes) and membrane proteins (outlined on membrane surface). (KEYSTONE/SCIENCE PHOTO LIBRARY/KEITH CHAMBERS/SCIENCE PHOTO LIBRARY)