Measles virus particle, illustration. This virus, from the Morbillivirus group of viruses, consists of an RNA (ribonucleic acid) core surrounded by an envelope studded with surface proteins haemagglutinin-neuraminidase and fusion protein, 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.