Measles virus particles. Coloured transmission electron micrograph (TEM) of a virus that causes measles (from the morbillivirus group of viruses). Each particle is surrounded by a lipoprotein envelope (purple), acquired from the host cell's cytoplasmic membrane as the virus buds off from the cell surface. The envelope is studded with surface proteins and encloses the nucleocapsid: a helical structure consisting of a single-stranded ribonucleic acid (RNA) core (genetic material). Measles is highly infectious and mainly affects children, producing fever and rash. One attack usually gives life-long immunity. Magnification: x144,000 when printed 10 centimetres high.