PERFECT STORM. “Since the size of RNA viral genomes lies between 3 kb and 33 kb, close to the inverse of the mutation rate, it is thought that it is unlikely to produce progeny RNAs identical to the parental genome even within the confined limits of a single replicative unit (each one of the replication complexes) within a single infected cell.”
http://www.sciencedirect.com/science/article/pii/S0042682215001580