Kwayar cutar syncytial na numfashi wata cuta ce ta RNA wacce ke cikin kwayar cutar Pneumovirus, dangin Pneumovirinae.Yana yaɗuwa ta hanyar watsa ɗigon ruwa, da kuma tuntuɓar yatsa kai tsaye da ƙwayar cuta ta syncytial ta numfashi tare da mucosa na hanci da mucosa na ido shima hanya ce mai mahimmanci ta watsawa.Kwayar cutar syncytial na numfashi shine sanadin ciwon huhu.Bayan lokacin shiryawa, ƙwayar cuta ta syncytial na numfashi za ta haifar da zazzaɓi, hanci, tari da kuma wani lokacin pant.Kwayar cutar syncytial na numfashi na iya faruwa a tsakanin al'ummomi na kowane rukuni na shekaru, inda manyan mutane da mutanen da ke da nakasa huhu, zuciya ko tsarin rigakafi suka fi kamuwa da cutar.