Originally used by Manfred Eigen to model the evolution of the first macromolecules on earth, the quasispecies concept has been applied to populations of a virus within its host. The quasispecies model is deemed to be relevant to RNA viruses because they have high mutation rates in the order of one per round of replication, and viral populations, while not infinite, are extremely large. Thus the practical conditions for quasispecies formation are thought to exist.
The significance of the quasispecies model for virology is that, if the mutation rate is sufficiently high, selection acts on clouds of mutants rather than individual sequences. Therefore, the evolutionary trajectory of the viral infection cannot be predicted solely from the characteristics of the fittest sequence.
The importance of quasispecies concepts in virology has been the subject of some discussion. Significantly, it has been shown that there is no necessary conflict between a quasispecies model of intra-host evolution and traditional population genetics. Instead, viral quasispecies can be considered as cases of coupled mutation-selection balance models for haploid organisms.
It may be useful to understand the etymology of the term. Quasispecies are clouds of related elements that behave almost (quasi) like a single type of molecule (species). There is no suggestion that a viral quasispecies resembles a traditional biological species.