Coronavirus and coronavirus-like infections are described in swine, cattle, horses, cats, dogs, rats, birds, bats, rabbits, ferrets, mink, and various wildlife species, although many coronavirus infections are subclinical or asymptomatic. In humans, coronaviruses are included in the spectrum of viruses that cause the common cold and, recently, severe acute respiratory syndrome (SARS), which is a zoonosis. This chapter explains that the family Coronaviridae is included with the families Arteriviridae and Roniviridae in the order nidovirales; viruses in these three families share a distinctive replication strategy. The genus Coronavirus contains a substantial number of pathogens of mammals and birds that individually cause a remarkable variety of diseases, including pneumonia, reproductive disease, enteritis, polyserositis, sialodacryoadenitis, hepatitis, encephalomyelitis, nephritis, and various other disorders. The genus Coronavirus can be subdivided into at least three cluster groups on the basis of genetic and serologic properties, with subgroups in two of these. Group 1a includes transmissible gastroenteritis virus of swine, porcine respiratory coronavirus, canine coronavirus, feline enteric coronavirus, ferret and mink coronaviruses, and spotted hyena coronavirus. Group 1b includes certain human coronaviruses, porcine epidemic diarrhea virus, and bat coronavirus. Group 2a includes mouse hepatitis virus, bovine coronavirus, sialodacryoadenitis virus of rats, porcine hemagglutinating encephalomyelitis virus, canine respiratory coronavirus, and other human coronaviruses. Group 2b includes human SARS coronavirus and civet cat, raccoon dog, and horseshoe bat coronaviruses. Group 3 includes avian infectious bronchitis virus, turkey coronavirus, and several potential but still largely uncharacterized new species from ducks, geese, and pigeons.