This thin-section transmission electron micrograph (TEM) revealed the ultrastructural appearance of a single virus particle, or ?virion?, of measles virus. The measles virus is a paramyxovirus, genus Morbillivirus. It is 100-200 nm in diameter, with a core of single-stranded RNA, and is closely related to the rinderpest and canine distemper viruses. Two membrane envelope proteins are important in pathogenesis. They are the F (fusion) protein, which is responsible for fusion of virus and host