Molecular model of the SARS-CoV-2 coronavirus spike (S) protein with the D614G mutation site highlighted (red). S proteins are found in the viral membrane, they bind to receptors on host cell membranes and facilitate the virus's entry to the cell. The D614G strain of the virus emerged in Europe and has become the predominant strain, suggesting that the mutation enhances transmission. SARS-CoV-2 emerged in Wuhan, China, in December 2019. The virus causes the disease Covid-19, a respiratory illness that can lead to pneumonia and can be fatal in some cases.