Illustration of coronavirus particles (blue spheres) infecting a human cell. The spike (S) protein (large protrusions) in the coronavirus membrane binds to angiotensin-converting enzyme 2 (ACE2) receptors (yellow and purple) in the human cell membrane, which facilitate the virus's entry to the cell (bottom right). Once inside the cell, the virus releases it RNA (ribonucleic acid) genome and 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 the ACE2 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.