Measles virus, a human pathogen, is in the genus Morbillivirus. It is an enveloped, single-stranded, non-segmented, negative-sense RNA virus. Its virions are pleomorphic (roughly spherical) in shape with a diameter of 300–500 nm. Parainfluenza and mumps viruses are within the genus Paramyxovirus, and the respiratory syncytial virus (RSV) and metapneumovirus are within the genus Pneumovirus.