Molecular model of a coronavirus spike (S) protein (red) bound to an angiotensin-converting enzyme 2 (ACE2) receptor (blue) on a human cell. Heparan sulfate proteoglycans (yellow) are acting as adhesion molecules, aiding the binding of the virus to the 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 cell's machinery to make more copies of itself. Both SARS (severe acute respiratory syndrome) and the new coronavirus SARS-CoV-2 (previously 2019-CoV) bind to this receptor. 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.