Measles virus. Transmission electron micrograph (TEM) of a measles virus (rubeola). The envelope of the virus has broken, exposing the nucleocapsid filaments. Nucleocapsids are protein-nucleic acid complexes which are normally coiled up into a ball. They contain the genetic material of the virus. Smaller simian viruses (SV40, white circles) are also seen, at upper right for example. These are polyoma viruses. Measles viruses are morbilliviruses, a type of paramyxovirus. Measles is highly infectious and mainly affects children, producing fever and a rash. One attack usually provides lifelong immunity.