A viral family made up of the small (18–30 nanometers) ether-sensitive viruses that lack an envelope and have a ribonucleic acid (RNA) genome. The name is derived from “pico” meaning very small, and RNA for the nucleic acid type. Most of the picornaviruses are stabilized by magnesium chloride against thermal inactivation. The virion is made up of a nucleic acid core surrounded by a capsid of 32 subunits (capsomeres) arranged in the icosahedral form of cubic symmetry. Within the infected cell, virus particles are assembled in the cytoplasm, where they tend to aggregate in crystalline array.