An order comprising three families of eukaryotic viruses possessing linear, nonsegmented, positive sense RNA genomes. The families are Mesoniviridae; CORONAVIRIDAE; ARTERIVIRIDAE; and RONIVIRIDAE.

Synonyms: Nidoviruses, Mesonivirus, Alphamesonivirus, Mesoniviridae, txid1312872, Nidovirus, txid76804, Mesoniviruses, Alphamesoniviruses, Nidovirales