Mechanisms of viral emergence

Domingo, Esteban

Viral emergences and re-emergences can be regarded as episodes of virus adaptation to a new environment. Adaptation can be measured as an increase of replicative fitness, defined as the capacity of a virus to produce infectious progeny in a given environment [11, 14, 71] . The term epidemiologic fitness describes in semiquantitative ways (for example, through diagnostic surveys, by sampling nucleotide sequences of viruses that compete in a given geographical area, etc.) the capacity of a virus (a serotype, clade or variant) to become dominant, relative to other serotypes, clades or variants of the same virus [14] . For example, a FMDVof serotype O termed FMDV O PanAsia was first isolated in India in 1990 and then, in the following years, the virus spread to several countries. FMDV PanAsia manifested high epidemiologic fitness, since it displaced other FMDV strains that circulated at that time in the same geographical areas [39] . A more recent example is provided by the new H1N1 human influenza virus that originated in swine, crossed the species barrier into the human population early in 2009, and is currently displacing the H1N1 and H3N1 viruses that were dominant at the time of irruption of the new H1N1 [74] . Replicative fitness is a relevant concept in evolutionary biology in general, and it has become very important to quantitate virus adaptability in numerous scenarios, including clinical and diagnostic virology (as reviews, see [11, 14, 71] ) (Fig. 1) .