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Illustration of the taxonomy tree for the Picornaviridae virus family. Virions with a known reconstructed structure (via cryogenic-electron microscopy or X-ray) are shown as 3D visualisations. Picornaviruses are very small single-stranded RNA (ribonucleic acid) viruses with an icosahedral capsid and without an envelope. They are human pathogens causing the common cold, poliomyelitis, meningitis, hepatitis and paralysis. (KEYSTONE/SCIENCE PHOTO LIBRARY/SIMONE ALEXOWSKI / SCIENCE PHOTO LIBRARY)