Paramyxovirus. Transmission electron micrograph (TEM) of a paramyxovirus particle. The internal structure of the virus has been revealed, including the helical structures (white) of the ribonucleic acid (RNA). The virus particle is a core of RNA surrounded by a lipid envelope (outer layer). Several members of the paramyxovirus group are disease-causing (pathogenic) viruses. These include the mumps virus, the measles virus (rubeola) and the respiratory syncytial virus (RSV), which causes respiratory infections.