Coronavirus host cell receptor with antibodies, illustration. Molecular model of an angiotensin-converting enzyme 2 (ACE2) type receptor (red), the entry route for all coronavirus particle types, in a host cell membrane (lower centre) binding to antibodies (blue). Antibodies are part of the immune system and help to fight disease. This receptor is a potential target for vaccines or therapies that block the receptor and make it harder for coronavirus to enter cells. 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 100,000 have been infected with several thousand deaths.