A family of RNA-containing viruses about twice the size of the influenza viruses (Orthomyxoviridae) but similar to them in morphology. Virions are 150–300 nm in diameter, enveloped and ether-sensitive and contain RNA-dependent RNA polymerase. Nucleocapsids are helical, considerably larger than those of the influenza viruses, and contain single-stranded unsegmented RNA. Four genera are recognized: Paramyxovirus, Morbillivirus, Rubulavirus, and Pneumovirus, all of which cause cell fusion and produce cytoplasmic eosinophilic inclusions. Diseases associated with these viruses include croup and other upper respiratory infections, measles, mumps, and pneumonia.
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Para·myxo·vi·ri·dae .par-ə-.mik-sə-'vir-ə-.dē n pl a family of large single-stranded RNA viruses that have a helical nucleocapsid and lipid-containing envelope and that include the parainfluenza viruses, respiratory syncytial virus, and the causative agents of canine distemper, measles, mumps, Newcastle disease, and rinderpest see morbillivirus, rubulavirus
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Para·myxo·vi·ri·dae (par″ə-mik″so-virґĭ-de) the paramyxoviruses: a family of RNA viruses having a pleomorphic, usually roughly spherical but occasionally filamentous virion 150–300 nm in diameter, consisting of a lipid bilayer membrane with large peplomers surrounding a helical nucleocapsid. The genome consists of a single molecule of negative-sense single-stranded RNA (MW 5–7 Ч 106, size 16 kb). Viruses contain six to ten major polypeptides, including a transcriptase, and are sensitive to lipid solvents, detergents, disinfectants, and extremes of pH; thermostability varies according to genus. Replication occurs in the cytoplasm and assembly is by budding through the plasma membrane. Host range is generally narrow in nature but broad in cultured cells, and transmission is horizontal, chiefly airborne. There are two subfamilies, Paramyxovirinae and Pneumovirinae.Medical dictionary. 2011.