For translation to occur, eukaryotic mRNAs require a 5' Cap structure. This means that viruses must also have one. This normally consists of 7MeGpppN where N is normally adenine or guanine. The viruses encode a protein, normally a replicase, with a methyltransferase activity to allow this.
Some viruses are cap-snatchers. During this process, a G-capped host mRNA is recruited by the viral transcriptase complex and subsequently cleaved by a virally encoded endonuclease. The resulting capped leader RNA is used to prime transcription on the viral genome.
However some plant viruses do not use cap, yet translate efficiently due to cap-independent translation enhancers present in 5 and 3 untranslated regions of viral mRNA.