Illustration of homologous recombination in coronavirus particles. The two different strains of coronavirus at right and left have infected a cell at the same time. Homologous recombination of their genomes within the cell has created a new strain of coronavirus (centre) with a mixture of genes from both original viruses. Such recombination is a possible origin of the new coronavirus SARS-CoV-2 (previously 2019-CoV) that 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.