MEASLES VIRUS (MeV) is a single-stranded RNA (ssRNA) virus (Morbillivirus) causing the childhood disease (sometimes leading to subacute sclerosing panencephalitis). Virions range in size, with the larger ones housing several genome copies (quite small particles can still be complete and infectious). Genome is spiral and apparently disordered, like a skein of rope, (although some virions seem more ordered). Matrix proteins (blue in graphic) surround the genome. The matrix helps virion assembly and orders the spikes. Surrounding the matrix is the lipid viral envelope acquired during budding from the host cell. The surface of the virion is covered in glycoprotein spikes (haemagglutinin and fusion).