shared in eating a fruit bat that carried the virus. What are the odds faced by bat-eaters? Hard to say; hard even to guess. If the hammer-headed bat is an Ebola reservoir, what’s the prevalence of the virus within a given population? That’s another unknown. Towner found 5 percent prevalence of Marburg in Egyptian fruit bats, meaning that one animal in twenty could be infected. Assuming a roughly similar prevalence in the hammer-headed bat, the little girl’s family had been unlucky as well as hungry. They might have eaten nineteen other bats and gotten no exposure. Then again, if a bat meal was shared, why didn’t the girl’s mother and other family members get sick? Possibly her father, infected or besmeared after purchasing bats in the market, had carried the girl (common practice with small children thereabouts) along the footpath back to their village. The father, patient C, seems to have passed the virus to nobody else.
But his little daughter did pass it along. Her dead body was washed for burial, in accord with local traditions, by a close friend of the family. That friend was the fifty-five-year-old woman who became patient A.
“Thus, virus transmission may have occurred when patient A prepared the corpse for burial ceremony,” Leroy’s group wrote. “When interviewed, the two other preparers, the girl’s mother and grandmother, reported they did not have direct contact with the corpse and they did not develop any clinical sign of infection in the four following weeks.” Their role in the funerary washing was apparently observational. They didn’t touch the dead body of their daughter and granddaughter. But patient A did, performing faithfully the service of a close family friend, after which she went back to her life—what was left of it. She resumed her social interactions, and 183 other people caught Ebola and died.
Leroy’s team reconstructed this story and then, keen to extract meaning, asked themselves several questions. Why had the father infected his daughter but no one else? Maybe because he had a mild case, with a low level of virus in his body and not much leaking out. But if his case was mild, why was his daughter’s so severe, killing her within four days? Maybe because, as a small child racked with vomiting and diarrhea, she had died of untreated dehydration. Why was there only one bat-to-human spillover event? Why was patient C unique, as the sole case linked directly to the reservoir? Well, maybe he wasn’t. He was just the only one that came to notice. “In fact, it is highly likely that several other persons were infected by bats,” Leroy’s group wrote, “but the circumstances required for subsequent human-to-human transmission were not present.” They were alluding to dead-end infections. A person sickens, suffers solitarily or with carefully distanced succor from wary family or friends (food and water left at the door of a hut), and dies. Is buried unceremoniously. Eric Leroy didn’t know how many unfortunate people in the Luebo area may have eaten a bat, touched a bat, become infected with Ebola, succumbed to it, and been dropped into a hole, having infected no one else. Amid the horrific confusion of the outbreak, in those remote villages, the number of such dead-end cases might have been sizable.
This brought Leroy’s team to the pivotal question. If the circumstances required for human-to-human transmission hadn’t been met, what were those circumstances? Why hadn’t the Luebo outbreak gone really big? Why hadn’t the tinder ignited the logs? It had started in May, after all, and WHO didn’t get there until October.
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Human-to-human transmission is the crux. That capacity is what separates a bizarre, awful, localized, intermittent, and mysterious disease (such as Ebola) from a global pandemic. Remember the simple equation offered by Roy Anderson and Robert May for the dynamics of an unfolding epidemic?
R0 = βN/(α + b + v)
In that formulation, β represents the transmission rate. β is the letter beta, in case you’re not a mathematician or a Greek. Here it’s a multiplier in the single expression that stands as numerator of the fraction, a strong position. What that means is, when β changes muchly, R0 changes muchly. And R0, your good memory tells you, is the measure of whether an outbreak will take off.
In some zoonotic pathogens, efficient transmissibility among humans seems to be inherent from the start, a sort of accidental preadaptedness for spreading through the human population, despite a long history of residence within some other host. SARS-CoV had it, from the earliest days