The family Picornaviridae is one of the largest families of viruses and includes a large number of very small ( pico: measuring small; rna: RNA virus) RNA viruses. They are nonenveloped viruses measuring 27–30 nm in size. The capsid is a naked icosahedral made up of 60 protein subunits. The genome consists of a single linear molecule of single-stranded RNA. The genome RNA is unusual, because it has a protein at the 59 end that serves as a primer for transcription by RNA polymerase.
The family Picornaviridae consists of more than 230 members and is divided into five genera: Enterovirus, Rhinovirus, Hepatovirus, Arthrovirus, and Cardiovirus. The first three genera(Enterovirus, Rhinovirus, and Hepatovirus) are the important viruses, which cause disease in humans. Two other genera (Arthrovirus and Cardiovirus) are of veterinary importance and cause foot-and-mouth disease in cattle and meningoencepha-lomyelitis in mice. The picornaviruses causing human infec-tions are summarized in Table 60-1. The important features of picornaviruses are summarized in Table 60-2.