RNA viruses replicate as complex and dynamic mutant distributions. They are termed viralquasispecies, in recognition of the fundamental contribution of quasispecies theory in ourunderstanding of error-prone replicative entities. Viral quasispecies have launched afertile field of transdiciplinary research, both experimental and theoretical. Here wereview the origin and some implications of the quasispecies concept, with emphasis oninternal interactions among components of the same mutant virus ensemble, a critical factto design new antiviral strategies. We make the distinction between “intrinsic” and“extrinsic” properties of mutant distributions, and emphasize that there are severallevels of complexity that can influence viral quasispecies behavior.
