Coronaviruses are a group of related viruses that cause diseases in mammals and birds. In humans, coronaviruses cause respiratory tract infections that can be mild, such as some cases of the common cold (among other possible causes, predominantly rhinoviruses), and others that can be lethal, such as SARS, MERS, and COVID-19. Symptoms in other species vary: in chickens, they cause an upper respiratory tract disease, while in cows and pigs they cause diarrhea.
Seven strains of human coronaviruses are known, of which four produce the generally mild symptoms of the common cold:
Human coronavirus OC43 (HCoV-OC43)
Human coronavirus HKU1
Human coronavirus NL63 (HCoV-NL63, New Haven coronavirus)
Human coronavirus 229E (HCoV-229E)
And three, with symptoms that are potentially severe:
Middle East respiratory syndrome-related coronavirus (MERS-CoV), previously known as novel coronavirus 2012 and HCoV-EMC
Severe acute respiratory syndrome coronavirus (SARS-CoV or “SARS-classic”)
Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), previously known as 2019-nCoV or “novel coronavirus 2019”
The coronaviruses HCoV-229E, -NL63, -OC43, and -HKU1 continually circulate in the human population and cause respiratory infections in adults and children world-wide.