Spillover: the powerful, prescient book that predicted the Covid-19 coronavirus pandemic.

by David Quammen

Frank Fenner guessed astutely that it was the dynamic between virulence and transmission. His tests of one grade versus another, using captive rabbits and captive mosquitoes, revealed that the efficiency of transmission correlated with the amount of virus available on a rabbit’s skin. More lesions, or lesions that lasted longer, meant more available virus. More virus smeared on mosquito mouthparts, more chance of transmission to the next rabbit. But “available virus” assumed that the rabbit was still alive, still pumping warm blood, and therefore still of interest to the vector. Dead, cold rabbits don’t attract mosquitoes. Between the two extreme outcomes of infection—healed rabbits and dead rabbits—Fenner found a point of balance. “Laboratory experiments showed that all field strains17 produced lesions that provided sufficient virus for transmission to occur,” he wrote. But the strains of very high virulence (grades I and II) killed rabbits “so quickly that infectious lesions were only available for a few days.” The milder strains (grades IV and V) produced lesions that tended to heal quickly, he added—and then the payoff, “whereas strains of grade III virulence were highly infectious for the lifetime of the rabbits that died and for a much longer period in those that survived.” Grade III, at that point, was still killing around 67 percent of the rabbits it touched. Myxoma virus, thirty years after its introduction, had found this level of virulence—being pretty damn leth...

Australian rabbit virus story: Virus lethality and evolutionary success