Molecular model of a coronavirus main protease (Mpro). 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. 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.