The quasispecies model is most applicable when the genome size is limited and the mutation rate is high, and so is most relevant to RNA viruses (including important pathogens) because they have high mutation rates (approx one error per round of replication), though the concepts can apply to other biological entities. In such scenarios, complex distributions of closely related variant genomes are subjected to genetic variation, competition and selection, and may act as a unit of selection. Therefore, the evolutionary trajectory of the viral infection cannot be predicted solely from the characteristics of the fittest sequence. High mutation rates also place an upper limit compatible with inheritable information. Crossing such a limit leads to RNA virus extinction, a transition that is the basis of an antiviral design termed lethal mutagenesis, and of relevance to antiviral medicine.