Molecular model of a coronavirus spike (S) protein (red) bound to an angiotensin-converting enzyme 2 (ACE2) receptor (blue) on a human cell. 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. Once inside the cell, the virus uses the cells' machinery to make more copies of itself. 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.