Covid-19 coronavirus spike proteins (ribbons), computer model. The new coronavirus SARS-CoV-2 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. The coronaviruses take their name from their crown (corona) of surface proteins. The spike (S) protein binds to a receptor on the host cell membrane and facilitate the virus's entry to the cell. Once inside a cell, the virus uses the cell's machinery to make more copies of itself. The viral surface (yellow) is based on cryoEM (electron microscope) density data.