explained, because of the character of the virus. It strikes rarely, it progresses quickly through the course of infection, it kills or it doesn’t kill within just a few days, it affects only dozens or hundreds of people in each outbreak, and those people generally live in remote areas, far from research hospitals and medical institutes—far even from his institute, CIRMF. (It takes about two days to travel, by road and river, from Franceville to Mayibout 2.) Then the outbreak exhausts itself locally, coming to a dead end, or is successfully stanched by intervention. The virus disappears like a band of jungle guerrillas. “There is nothing to do,” Leroy said, expressing the momentary perplexity of an otherwise patient man. He meant, nothing to do except keep trying, keep working, keep sampling from the forest, keep responding to outbreaks as they occur. No one can predict when and where Ebola virus will next spill. “The virus seems to decide for itself.”
21
The geographical pattern of Ebola outbreaks among humans is, as I’ve mentioned, controversial. Everyone knows what that pattern looks like but experts dispute what it means. The dispute involves Ebola virus in particular, the one among those five ebolaviruses that has emerged most frequently, in multiple locations across Africa, and therefore cries most loudly for explanation. From its first known appearance to the present, from Yambuku (1976) to Tandala (1977) to the upper Ivindo River gold camps (1994) to Kikwit (1995) to Mayibout 2 (1996) to Booué (later 1996) to the northern border region between Gabon and the Republic of the Congo (2001–2002) to the Mbomo area (2002–2003) to its recurrence at Mbomo (2005) and then to its two more recent appearances near the Kasai River in what’s now the Democratic Republic of the Congo (2007–2009), Ebola virus has seemingly hopscotched its way around Central Africa. What’s going on? Is that pattern random or does it have causes? If it has causes, what are they?
Two schools of thought have arisen. I think of them as the wave school and the particle school—my little parody of the classic wave-or-particle conundrum about the nature of light. Back in the seventeenth century, as your keen memory for high-school physics will tell you, Christiaan Huygens proposed that light consists of waves, whereas Isaac Newton argued that light is particulate. They each had some experimental grounds for believing as they did. It took quantum mechanics, more than two centuries later, to explain that wave-versus-particle is not a resolvable dichotomy but an ineffable duality, or at least an artifact of the limitations of different modes of observing.
The particle view of Ebola sees it as a relatively old and ubiquitous virus in Central African forests, and each human outbreak as an independent event, primarily explicable by an immediate cause. For instance: Somebody scavenges an infected chimpanzee carcass; the carcass is infected because the chimp itself scavenged a piece of fruit previously gnawed by a reservoir host. The subsequent outbreak among humans results from a local, accidental event, each outbreak therefore representing a particle, discrete from others. Eric Leroy is the leading proponent of this view. “I think the virus is present all the time, within reservoir species,” he told me. “And sometimes there is transmission from reservoir species to other species.”
The wave view suggests that Ebola has not been present throughout Central Africa for a long time—that, on the contrary, it’s a rather new virus, descended from some viral ancestor, perhaps in the Yambuku area, and come lately to other sites where it has emerged. The local outbreaks are not independent events, but connected as part of a wave phenomenon. The virus has been expanding its range within recent decades, infecting new populations of reservoir in new places. Each outbreak, by this view, represents a local event primarily explicable by a larger cause—the arrival of the wave. The main proponent of the wave idea is Peter D. Walsh, an American ecologist who has worked often in Central Africa and specializes in mathematical theory about ecological facts.
“I think it’s spreading from host to host in a reservoir host,” Walsh said, when I asked him to explain where the virus was traveling and how. This was another conversation in Libreville, a teeming Gabonese city with pockets of quietude, through which all Ebola researchers eventually pass. “Probably a reservoir host that’s got large population sizes and doesn’t move very much. At least, it doesn’t transmit the virus very far.” Walsh didn’t claim to know the identity of that reservoir, but it