A coronavirus is one of many viruses that cause diseases in mammals and birds. In humans, coronaviruses cause respiratory tract infections that are typically mild, such as the common cold, though rarer forms such as SARS, MERS and COVID-19 can be lethal. Symptoms vary in other species: in chickens, they cause an upper respiratory tract disease, while in cows and pigs they cause diarrhea. There are no vaccines or antiviral drugs to prevent or treat human coronavirus infections.
Coronaviruses comprise the subfamily Orthocoronavirinae in the family Coronaviridae, in the order Nidovirales. They are enveloped viruses with a positive-sense single-stranded RNA genome and a nucleocapsid of helical symmetry. The genome size of coronaviruses ranges from approximately 27 to 34 kilobases, the largest among known RNA viruses. The name coronavirus is derived from the Latin corona, meaning "crown" or "halo", which refers to the characteristic appearance of the virus particles (virions): they have a fringe reminiscent of a crown or of a solar corona.