Measles virus particle, coloured transmission electron micrograph (TEM). This virus, from the Morbillivirus group of viruses, consists of an RNA (ribonucleic acid) core surrounded by an envelope studded with surface proteins, which are used to attach to and penetrate a host cell. Measles is a highly infectious itchy rash with a fever. It mainly affects children, but one attack usually gives life-long immunity.