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"Adenovirus. Coloured transmission electron micrograph of adenovirus particles (type not identified) cause of repiratory tract infections including the common cold. The viruses are about 80nm in diameter, and do not have the lipoprotein envelope associated with many other virus genera. The genome of DNA is enclosed within a capsid of icosahedral symmetry; the particles appear hexagonal with triangular faces visible. Close packing (as at centre) may be found; gaps between these viruses may be due to shrinkage during preparation, to local repulsive forces or to the presence of apical fibres. Magnification: x24,000 at 6x4.5cm size." (KEYSTONE/SCIENCE PHOTO LIBRARY/BONNIE COSGROVE)