Molecular model of the main protease (Mpro) of the Covid-19 coronavirus complexed with an inhibitor. Proteases are enzymes that break proteins into smaller polypeptides. This protease processes two long polyproteins that the virus needs to replicate new viruses. Therefore Mpro inhibitors could be effective drugs against the virus. Covid-19, which emerged in Wuhan, China, in December 2019, is a mild respiratory illness that can develop into pneumonia and be fatal in some cases. It is caused by the SARS-CoV-2 (previously 2019-nCoV) coronavirus. During January 2020, the virus spread throughout China and internationally. As of early February there have been over 24,000 confirmed cases and more than 490 deaths in China, with another 191 cases and one death internationally.