Members of the family Coronaviridae are large, enveloped, single-stranded RNA viruses. They are the largest known RNA viruses, with genomes ranging from 25 to 32 kb and virions of 118–140 nm in diameter. The family is divided into two subfamilies, the Coronavirinae and the Torovirinae. They can be distinguished on the basis of their nucleocapsids as the toroviruses have unique doughnut-shaped nucleocapsids. Virions are roughly spherical and are notable for the large spike (S) glycoprotein that extends from the virus envelope. Current taxonomy places the family in the order Nidovirales. Within the subfamily Coronavirinae are four genera, the alpha-, beta-, gamma-, and deltacoronaviruses. All family members share the same unique strategy for mRNA synthesis whereby the polymerase complex jumps or moves from one region of the template to a more distant region. The need for the polymerase complex to dissociate from the template may explain the high rate of RNA recombination that occurs during genome replication. Both the coronaviruses and toroviruses are enteric and respiratory tract pathogens, usually associated with only mild disease (or inapparent infection). However the human severe acute respiratory syndrome (SARS) coronavirus and the Middle East respiratory syndrome (MERS) coronavirus cause severe respiratory diseases.