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set of rabies virus strains—thirty-six from dogs and seventeen from bats—and the results were fairly clear: the enigmatic bat, a distant presence for most of the cultural history of rabies, was probably responsible for infecting the dog, rather than the other way around.
    This so-called molecular clock research has led to many other insights about the origins of disease. In particular, it’s shown us how many of our worst killers, pathogens that have racked humanity since the earliest civilizations, evolved out of animal populations. Measles, we now know, evolved from a disease in cattle; similarly, the various strains of influenza, as we still see today in our annual flu scares,readily pass back and forth between us and our livestock (for more on this, see Chapter 6 ). Some of these zoonotic leaps from animal to man have been understood fully only during the past decade or so, as genome sequencing has allowed scientists to trace more precisely the genetic lineage of pathogens. For example, a team led by the Stanford epidemiologist Nathan Wolfe announced in 2009 that it had isolated the origins of malaria in a parasite of chimpanzees, which presumably spread to humans through mosquito bites.
    New sleuthing has yielded particularly intriguing details about smallpox, arguably the deadliest disease in history. A 2007 study, headed up by researchers at the U.S. Centers for Disease Control and Prevention, traced the notorious killer back to a virus in rodents, estimating that it made the leap to humans at least sixteen thousand years ago. What is especially satisfying is the team’s identification of two separate human strains, an earlier and milder version that cropped up in west Africa and the Americas, and a more severe version—the progenitor of the strain that slew untold millions over the past millennium before its eradication in the late 1970s—that emerged from Asia a bit later. This helps explain why the literature, medical and otherwise, of the Greeks and Romans provides little evidence that highly fatal smallpox was common, even though archaeological evidence shows the clear presence of a smallpox-like condition in ancient Egypt. The most spectacular example of this is the mummified body of Pharaoh Ramses V, on whose shriveled skin can clearly be seen the pustular pattern typical of the disease. (This possibly answers the vexing Egyptological question of why Ramses V was not buried for almost two years after his death, when other pharaohs were interred just seventy days after mummification; either fear of infection from his corpse or a paucity of healthy embalmers might account for the lag.)
    Smallpox was far from the only ancient epidemic with its origins in the rodent. Both plague and typhus ravaged by way of the rat, whosefleas would transmit the deadly bugs to unsuspecting humans. For all the emphasis placed on livestock in the development of civilization, the case can be made—and indeed has been made, most elegantly by the biologist Hans Zinsser in his 1935 book
Rats, Lice, and History
—that human affairs have been stirred far more vigorously by the rat, whose companionship with people has tended to be involuntary on our part but whose omnipresence among us, like that of the stray dog, became more or less inevitable with the emergence of the city. With most zoonotic leaps in disease, animal contact is the spark, but urbanization is the bone-dry tinder; a newly evolved pathogen can’t spread from person to person, after all, unless people run across one another in the first place.
    How to treat the rabies patient or the dog-bite victim? Consider the predicament of an ancient physician on this terrible question. The cause of hydrophobia (the bite of a rabid animal) was often separated by many weeks from its effect (the onset of neurological symptoms), and only a fraction of bites—even assuming an animal that is actually rabid and not merely vicious—progressed to the fatal infection. Meanwhile, it was