The plus-strand RNA [(+)RNA] viruses comprise a very large group of viruses belonging to many families. Among these are viruses that cause epidemic disease in humans, including encephalitis, hepatitis, polyarthritis, yellow fever, dengue fever, poliomyelitis, and the common cold. The number of cases of human disease caused by these viruses each year is enormous. As examples, dengue fever afflicts more than 100 million people each year; most humans suffer at least one rhinovirus-induced cold each year, with the cases therefore numbering in the billions; and most humans during their lifetime will suffer several episodes of gastroenteritis caused by astroviruses or caliciviruses. In terms of frequency and severity of illness, the (+)RNA viruses contain many serious human pathogens, and we will begin our description of viruses with this group.

The human (+)RNA viruses belong to six families (Table 3.1). These six families also contain numerous nonhuman viruses, of which many are important pathogens of domestic animals. Large numbers of (+)RNA viruses that infect plants are also known; in fact, most plant viruses contain (+)RNA genomes. The plant viruses, however, belong to different families and are currently classified by the International Committee on Taxonomy of Viruses (ICTV) into six families plus many unassigned genera. Because of their importance as disease agents of domestic crops, much is known about these viruses. Other families of (+)RNA viruses include a family of bacterial viruses and two families of insect viruses (the nodaviruses, in particular, have been intensively studied). Thus, the (+)RNA viruses have evolved into many distinctly different families and must have arisen long ago. In this chapter, the six families of viruses that include human viruses as members are

TABLE 3.1 Families of Plus-Strand RNA Viruses That Contain Human Pathogens


Size of genome

Other vertebrate

Representative human pathogens"

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