The viruses which contain positive strand RNA genome act directly as mRNA upon infection to a host cell. Most viruses in this category have icosahedral symmetry and vary approximately 50-150 nm in diameter. The members contain positive sense RNA genome containing either lipid envelope ( Togaviridae , Flaviviridae , and Coronaviridae ) or devoid of envelope ( Picornaviridae ). They are important because they can cause serious life-threatening diseases including hemorrhagic fever and encephalitis (dengue, yellow fever). Members of genus Alphavirus (Sindbis virus, Semiliki Forest virus, and Equine encephalitis) are transmitted to animal by mosquito bite. Among all Alphaviruses sindbis virus is well studied and understood.

Table 17.1 Examples of positive strand RNA viruses: