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 genome and a nucleocapsid of helical symmetry. The genome size of coronaviruses ranges from approximately 26 to 32 kilobases , the largest for an RNA virus .The name "coronavirus" is derived fr...