FIG 1.

Opportunities for change in a given viral population arise both in vivo and in vitro. (A) The viral population in an infected individual (represented by red or red-shaded virion) may change over time due to immune selection or the accumulation of genetic drift. Bottlenecks at transmission to a new host or during introduction to tissue culture may allow a new genotype to become prevalent, akin to a genetic shift. (B) A viral stock grown in culture is also a viral population, which may undergo changes during introduction to an animal model or through plaque purification. See Fig. 2 for an expanded view of the genomes contained in the viral population.