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Coloured scanning electron micrograph (SEM) of SARS-CoV-2 coronavirus particles (yellow) on a cell from a US patient sample. SARS-CoV-2 (previously 2019-nCoV) was first identified in Wuhan, China, in December 2019, and has been declared a pandemic. It is an enveloped RNA (ribonucleic acid) virus. It causes Covid-19, a respiratory infection that can lead to fatal pneumonia. Social restrictions and infection control measures have been put in place to slow the spread of the virus. (KEYSTONE/SCIENCE PHOTO LIBRARY/NIAID/NATIONAL INSTITUTES OF HEALTH/SCIENCE PHOTO LIBRARY)