Pneumonia virus of mice (PVM), human respiratory syncytial virus (hRSV) and bovine respiratory syncytial virus (bRSV) are enveloped, negative sense, single-stranded RNA viruses of the family Paramyxoviridae, subfamily Pneumovirinae, genus Pneumovirus. PVM was originally discovered in 1939 by researchers Horsfall and Hahn at The Rockefeller University as part of an attempt to identify pathogens from human clinical samples that would replicate in lung tissues of inbred mice. PVM was isolated from lung tissue of what had been presumed to be healthy control mice that had been subjected to serial mouse-to-mouse passage. PVM virions are polymorphic and found in diverse shapes, from spheres of 80–120 μm in diameter to filaments up to 3 μm in length. The virus replicates over a period of 24–30 hours in mouse lung tissue, with virus amplification proceeding at 16-fold per cycle.

It is not yet clear how or if PVM replicates and induces pathology in non-rodent hosts. In a study carried out in 1986, Pringle and Eglin found that more than 75% of adult sera had PVM‑neutralizing activity that did not correlate with hRSV or parainfluenza virus (PIV)-3 neutralizing activity. More recently, Brock and colleagues explored this question further, and determined that PVM did not replicate in situ when administered to the respiratory tracts of non-human primates, and that the PVM-neutralizing factor(s) in human sera did not interact specifically with virion components. In another recent development, Dubovi and colleagues reported the isolation of canine pneumovirus (CnPnV) from the respiratory tracts of shelter-confined dogs with apparent respiratory illness.