Coronaviruses are a group of viruses that usually cause mild illnesses, such as the common cold. However, certain types of coronavirus can infect the lower airway, causing serious illnesses like pneumonia or bronchitis. Most people get infected with coronaviruses at some point in their lives and the majority of these infections are harmless. The new coronavirus that causes the covid-19 illness is a notable exception.
Coronaviruses have extraordinarily large single-stranded RNA genomes – approximately 26,000 to 32,000 bases or RNA “letters” in length. Coronavirus particles are surrounded by a fatty outer layer called an envelope and usually appear spherical, as seen under an electron microscope, with a crown or “corona” of club-shaped spikes on their surface.