Measles virus. Coloured transmission electron micrograph (negative stain) of a measles virus (of the morbillivirus group). The lipoprotein envelope of the virus is acquired from the host cell's cytoplasmic membrane as the virus buds off from the cell surface. The envelope encloses the nucleocapsid: a helical structure consisting of a single-stranded RNA core (genetic material) surrounded by protein. Measles is highly infectious and mainly affects children, producing fever and rash. One attack usually gives life-long immunity. Magnification: x90,000 at 6x4.5cm size.