Molecular model of antibodies (blue) binding to the spike (S) protein (red) of the new coronavirus SARS-CoV-2. The virus emerged in Wuhan, China, in December 2019, and causes a mild respiratory illness (covid-19) that can develop into pneumonia and be fatal in some cases. As of mid April 2020, over 1.7 million have been infected with over 111,500 deaths. The coronaviruses take their name from their crown (corona) of surface proteins, which are used to attach and penetrate their host cells. Once inside the cells, the particles use the cells' machinery to make more copies of the virus. Antibodies bind to specific antigens, for instance viral proteins, marking them for destruction by other immune cells.