A virus with an RNA genome that may be either an mRNA, (+)-RNA, or its complement, (-)-RNA. Class 1 contains (+)-RNA; class 2, (-)-RNA, which is the template for an RNA-dependent RNA polymerase; class 3, double-stranded RNA, in which (+)-RNA is synthesised by an RNA-dependent RNA polymerase; class 4, retrovirus, in which (+)-RNA is a template for an RNA-dependent DNA polymerase (a reverse transcriptase).