Measles virus. Coloured transmission electron micrograph (TEM) of a measles virus (of the morbillivirus group). A small piece of viral nucleoprotein (yellow), part of the nucleocapsid, can be seen to the upper left of the virus particle. The lipoprotein envelope (blue) 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: x105,000 at 10 centimetres tall.