Illustration of the SARS-CoV-2 coronavirus spike (S) proteins (purple) binding to angiotensin-converting enzyme 2 (ACE2) (orange) on a target cell. Initial spike protein priming by transmembrane protease, serine 2 (TMPRSS2, blue) allows the fusion of the viral membrane with the cell membrane and the entry of the viral RNA (ribonucleic acid, green, helical) into the cell . SARS-CoV-2 emerged in Wuhan, China, in December 2019 and spread throughout the world. The virus causes a mild respiratory illness (Covid-19) that can develop into pneumonia and be fatal in some cases. For a labelled version of this image see C050/6227.