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