False-colour scanning electron micrograph (SEM) of a group of adenoviruses (yellow) on the surface of a red blood cell of a chicken. Adenoviruses are the causative agents of many respiratory diseases. The viruses are about 80 nanometres in diameter, and do not have the lipoprotein envelope associated with many other virus genera. The genome of the adenovirus is double-stranded and linear DNA. Adenoviruses were first isolated from human adenoidal glands. Magnification x5700 at 6x7cm size.