Any of a group of the smallest known animal
( Pico refers to their small size, rna to their core of RNA .) This group of spheroidal viruses includes viruses that attack the vertebrate intestinal tract and often invade the central nervous system as well (such as polioviruses), viruses that infect tissues in the vertebrate nose ({{link=rhinovirus">rhinoviruses ), and the virus agent of foot-and-mouth disease .