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recent litter; Sylvia has “lately been gored by a boar.” Some thirty-five dogs are noted by name, with “many more too numerous to mention,” all dogs he has raised and fed; now they chargetoward him in a slavering mob, “out to taste his blood.” It is hard to know which of these twinned faces of
lyssa
is more horrible, in either Ovid’s reckoning or ours: the human becoming animal, or the hunter being hunted by his own treasured dogs.
    Perhaps the most enduring ancient symbol of the dog’s two warring natures is Cerberus, that terrifying watchdog whose vigilant gaze and fearsome jaws kept the dead from escaping Hades and returning to the world of the living. Descriptions of his physiology vary significantly in the different retellings—his heads number two, sometimes three, sometimes fifty, or even a hundred; his tail is that of a snake, or not; snake heads sometimes sprout from his head and neck like a gruesome mane. But despite all these monstrous innovations he is consistently described as a dog. A “cursed” or “dreaded” or “savage” dog he may be, but he remains a dog nonetheless, the unmistakable kin of those that walk the earth and lick its inhabitants. He could even be a good dog, at times. As described by Hesiod, Cerberus was quite friendly to the dying, at least when they arrived; he positively welcomed them, in fact, “with actions of his tail and both ears.” It was only when they attempted to pass
back
into life that he would set upon them savagely, even devour them. Death is a boundary that can be freely crossed in only one direction, and so guarding that boundary is a perfect role for a dog: natural friend on the one hand—or head; savage attacker and corpse devourer on the other; both natures cohabiting inside one vexing four-footed form.
    It was more than just the power of Cerberus’s many jaws that was to be feared. In the
Metamorphoses,
a list of poisonous substances includes “slaver from Cerberus,” along with a creation myth whereby that rabid saliva, sprayed from the hellhound’s lips and flecking a field of battle, gave rise to a notoriously poisonous plant called aconite—also known, tellingly, as wolfsbane. As the veterinary historian John Blaisdell has noted, symptoms of aconite poisoning in humans bear some passing similarity to those of rabies: they can include frothy saliva, impaired vision, vertigo, and finally a coma. It is not improbable that some ancient Greeks would have believed that this poison,mythically born of Cerberus’s lips, was literally the same as that to be found inside the mouth of a rabid dog.
    Until just the past century—and even then only in the developed world—rabies has been experienced by humans as a disease of the dog, a peculiarly canine madness that could reproduce a similar, fatal madness in humans. But all the while, the disease also lurked inside another, far more shadowy species: the bat. Indeed, recent research has indicated that bats harbored the disease even earlier than dogs, going back at least seven thousand years and as far as twelve thousand years, far before the first written languages and perhaps even before dogs were domesticated from wolves.
    How was this calculation made? The answer flows from two simple facts about how viruses evolve over time. The first is that most mutations in a virus are neither beneficial nor harmful to its propagation; instead, they’re neutral, trivially altering the genetic sequence without changing the virus’s overall fitness in any way. The second fact is that these mutations tend, over large populations and long periods of time, to happen on a predictable schedule. So given a set of related viral strains, a computer can analyze the patterns of genetic difference and arrange them into a rough phylogenetic tree, showing which strain evolved from which and how long ago the divergences occurred. In 2001, two researchers at France’s Institut Pasteur used this technique to investigate a large