> The question Fenner asked was: What happens when such a virulent virus spreads through a very susceptible host species on a continental scale? He focused on two possibilities. First, the highly lethal virus might evolve to become less lethal. Second, the highly susceptible rabbits might evolve resistance. Thanks to Fenner, we now know both happened.

> The work showed that the almost invariably lethal progenitor virus strain was replaced within a few years by strains with case fatality rates of 70 percent to 95 percent. Some field isolates killed fewer than half the lab rabbits. Over the next few decades, things settled down, and strains at both ends of the lethality spectrum become increasingly difficult to find. Fenner showed why. The highly lethal progenitor virus killed rabbits so fast that its infectious period was shorter than that of the less lethal viral mutants. That meant that the less lethal strains were able to infect more new victims and spread throughout the population.