Coronavirus particle, illustration. The coronaviruses are composed of an RNA (ribonucleic acid, helical) core surrounded by an envelope (light blue) containing surface proteins (red and blue). Spike proteins (dark blue) are used to attach and penetrate their host cells. Once inside the cells, the particles use the cells' machinery to make more copies of the virus. Different strains of coronavirus are responsible for diseases such as the common cold, gastroenteritis and SARS (severe acute respiratory syndrome). The new coronavirus SARS-CoV-2 (previously 2019-CoV) emerged in Wuhan, China, in December 2019. The virus causes a mild respiratory illness (Covid-19) that can develop into pneumonia and be fatal in some cases.