A coronavirus is one of a number of viruses that cause diseases in mammals and birds. In humans, the viruses cause respiratory infections, including the common cold, which are typically mild, 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 disease, while in cows and pigs coronaviruses 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.[5][6] 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 26 to 32 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.

In 2019, an outbreak began in Wuhan, China, and is ongoing as of February 2020. As of 15 February it has affected over 66,000 people. Because of the virus's contagiousness, mortality rate, and resistance to treatment, it has prompted extensive government action including mandated quarantines. Internationally, countries have limited travel to and from affected regions and implemented screening measures to detect potential carriers of the virus.