A picornavirus is a virus belonging to the family Picornaviridae. Picornaviruses are non-enveloped, positive-stranded RNA viruses with an icosahedral capsid. The genome RNA is unusual because it has a protein on the 5' end that is used as a primer for transcription by RNA polymerase. The name is derived from pico meaning small, and RNA referring to the ribonucleic acid genome, so "picornavirus" literally means small RNA virus. Picornaviruses are separated into 12 distinct genera and include many important pathogens of humans and animals. The diseases they cause are varied, ranging from acute "common-cold"-like illnesses, to poliomyelitis, to chronic infections in livestock.