any of a group of viruses of the families Orthomyxoviridae (agents of influenza) and Paramyxoviridae, members of which can cause the common cold, mumps, and measles in humans, canine distemper, rinderpest in cattle, and Newcastle disease in fowl. The virus particle is enveloped in a fatty membrane; is variable in shape, from spheroidal to filamentous, and in size, from 60 to 300 nanometres (1 nanometre = 10^-9 metre) in longest dimension; is studded with spikelike protein projections; and contains ribonucleic acid (RNA). These viruses react with mucin (mucoprotein) on the surface of red blood cells (hence the prefix myxo-, Greek for “mucin”); many of them cause red cells to clump together (agglutinate). Compare retrovirus.