Illustration of the SARS-CoV-2 coronavirus. SARS-CoV-2 emerged in Wuhan, China, in December 2019. Within the viral membrane (yellow) are spike proteins (blue), membrane proteins (yellow) and envelope proteins (pink). The spike proteins bind to angiotensin-converting enzyme 2 (ACE2) receptors on human cells to infect them. The virus causes the disease Covid-19, a respiratory illness that can lead to pneumonia and can be fatal in some cases.