Coronaviruses are enveloped, positive-strand RNA viruses with genomes approximately 30 kb in length that belong to the family Coronaviridae in the order Nidovirales. Coronaviruses infect a wide variety of mammalian and avian species, in most cases causing respiratory and intestinal tract disease. Human coronaviruses (HCoVs), such as HCoV-OC43, HCoV-NL63 and HKU1, have long been recognized as major causes of the common cold.