Illustration of a SARS-CoV-2 coronavirus particle infecting a human cell. The spike (S) protein (large protrusions) in the coronavirus membrane binds to angiotensin-converting enzyme 2 (ACE2) receptors in the human cell membrane, which facilitate the virus's entry to the cell. Once inside the cell, the virus releases its RNA (ribonucleic acid) genome and uses the cell's machinery to make more copies of itself. SARS-CoV-2 (previously 2019-CoV) 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.