Transmission electron micrograph (TEM) of parainfluenza virus. Two intact particles and free filamentous nucleocapsid. Human parainfluenza viruses (HPIVs) are the viruses that cause human parainfluenza. HPIVs are a group of four distinct single-stranded RNA viruses belonging to the Paramyxoviridae family. These viruses are closely associated with both human and veterinary disease. The viruses can be detected via cell culture, immunofluorescent microscopy, and PCR. HPIVs remain the second main cause of hospitalization in children under 5 years of age suffering from a respiratory illness.