Covid-19 coronavirus binding to target cell, illustration. Molecular model of the Covid-19 coronavirus (yellow sphere) and the spike proteins (blue) used to bind to receptors (purple) on the target cell. In the background are the cilia found in cells such as those in the respiratory system. The receptor is an angiotensin-converting enzyme 2 (ACE2) type receptor, which is the entry route for all coronavirus particle types. Covid-19, which emerged in Wuhan, China, in December 2019, is a mild respiratory illness that can develop into pneumonia and be fatal in some cases. It is caused by the SARS-CoV-2 (previously 2019-nCoV) coronavirus. As of March 2020, the virus has spread to many countries worldwide and has been declared a pandemic. Over 200,000 have been infected with thousands of deaths.