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. of the 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 cell membranes. viral penetration. and hemolysis. and the H (hemagglutinin) protein. which is responsible for adsorption of virus to cells. There is only one antigenic type of measles virus. Although studies have documented changes in the H glycoprotein. these changes do not appear to be epidemiologically important (i.e.. no change in vaccine efficacy has been observed).