Measles virus particle, illustration. This virus, from the Morbillivirus group of viruses, consists of an RNA (ribonucleic acid) core surrounded by a envelope studded with the surface proteins haemagglutinin (red) and fusion protein (grey). Haemagglutinin plays a role in the virus attaching to host cells, while the fusion protein is involved in the fusion with and penetration of host cells. Measles is a highly infectious itchy rash with a fever. It mainly affects children, but one attack usually gives lifelong immunity.