Kisses are at a premium in the capital of Liberia. Even a hug or a handshake between friends is often out of the question. That’s the new normal ever since Ebola began ravaging communities throughout Liberia, Sierra Leone and Guinea. For much of the past year, residents of these west African countries have wondered if daily life will ever be able to return to the way things once were. And at the heart of the matter is a scientific question: has Ebola now found a permanent foothold among humans? The answer, however, is not easy to suss out. In fact, it’s a guessing game. Even for top scientists.



In public health terminology the word used to describe this kind of health threat is “endemic.” The term describes any malady that routinely crops up without having to be reintroduced from an outside source—either imported from another country or another species. The flu, for example, is endemic in the U.S. because various strains reappear the following year without any trouble. Yet the many Ebola outbreaks of the past 40 years are not referred to as endemic because the original source of the infection in each case is widely believed to be an animal that somehow infected a human.



Changing the technical description of the current outbreak from epidemic to endemic is more than a matter of semantics. The difference between responding to an epidemic versus an endemic disease is as great as the difference between preparing for a sprint versus a marathon. A sprint requires a massive surge of effort after which the runner can recover. A marathon, like endemic Ebola, requires an entirely different mindset and extensive resources to go the distance. Failure to prepare for a marathon leaves a runner puffing shortly into the race. And failure to prepare for endemic Ebola results in a higher body count than might otherwise occur. But a premature shift to prepping for endemic Ebola could also result in a higher body count by crippling the short-term response; it would rob responders of the emergency beds and equipment needed to tamp down the massive viral surge still plaguing west Africa. Consequently, wary top health officials must draw up blueprints for the current crisis while eyeing the unpredictable road ahead.



When it comes to Ebola, “To say it is endemic is, in one sense, to admit failure,” says Christopher Dye, who serves as director of strategy in the office of the director general at the World Health Organization. “Our goal, and our expectation, is that we will eliminate infection from the human population,” he says. But there is no firm cutoff for a time period or series of symptoms that would demarcate the line between Ebola transmission as a perpetual threat or just a virus that is taking too long to extinguish.



From the time Ebola was first recognized in 1976 until this past year the virus never managed to gain much ground. All of the prior outbreaks were located in such remote areas that the combination of fast action and the relative isolation of the communities allowed the outbreaks to remain contained. But following that same strategy in the current case was impossible because the outbreak occurred in the more populous intersection of three countries and quickly escalated to dwarf every earlier Ebola outbreak. Left unchecked, Ebola would have been even more devastating for west Africa and beyond. But since Ebola still managed to ravage so many communities in west Africa its longevity raises questions about when or if Ebola will be considered endemic.



But where did Ebola come from in the first place? The virus did not appear out of thin air. Most virologists think the outbreaks are the result of a spillover from one or more animals that naturally carry the virus. One leading theory is that humans have contracted Ebola by consuming infected fruit bats. Multiple research groups have theorized bats are behind the disease, partly because a closely related malady, Marburg disease, has been linked to bats. Endemic Ebola, however, could cut out the need to encounter an infected animal altogether. Instead, Ebola would continue to readily spread between humans since there would always be low levels of the virus in the population.



Dye first sounded the alarm of a future with endemic Ebola in a New England Journal of Medicine article in September. For the first time, he wrote, scientists must “face the possibility that [Ebola Virus Disease] will become endemic among the human population of west Africa, a prospect that has never previously been contemplated.” In a recent interview with Scientific American he spelled out what he meant: “I think the reason we have used the word endemic in the first instance is to emphasize that the persistence of transmission has been a lot longer than anything we’ve seen before,” he says. But it could also be used to point to the need for an entirely different kind of response, one that would hinge on addressing the virus beyond an exponential growth phase, “where we get the virus to low levels in the population and there will be a different kind of response. Then we might use the word endemic there too,” he says.



Ebola expert Daniel Bausch, who has worked to quash Ebola during planning sessions in Geneva and on the ground in west Africa, unequivocally says that the epidemic that has led to more than 20,000 cases and 7,000 deaths is not at risk of becoming endemic in humans. Endemic Ebola, he says, would involve “long-standing perpetual transmission of Ebola virus in the area.” And although Ebola has ravaged west Africa since early 2014, the virus, he says, is on the correct path to be stamped out. An area would be considered Ebola-free after no new cases of Ebola appear for 42 days, twice the maximum incubation period for Ebola virus disease. Ebola may still crop up sporadically in the years to come, Bausch says, but “I think ultimately we will eventually get a handle on this, wait 42 days and call this outbreak over, so it is not fair to consider it endemic.”



Yet grappling with how to get answers to this endemic question through knowable, testable research is murky at best. As Bausch says, “What’s the difference between a big long outbreak that takes a long time to control and endemic disease?” The very characteristics of Ebola that make it so lethal also simultaneously block it from becoming a strong candidate to be endemic. Since Ebola kills pretty readily, for example, it doesn’t have the opportunity like HIV to pass itself on. And there’s no chronic carrier of this virus who appears to harbor the virus even after it has been eliminated from a community. Ebola can take months to be cleared from certain protected sites in the body like the gonads, but that’s not like HIV, which has true abilities to survive for years in the body and mount a resurgence if a patient stops taking medications to suppress the amount of virus circulating in the body.



Genetic sequencing can allow scientists to start answering questions about where the virus is coming from – say if Ebola was clearly just being passed from one person to the next or if the virus was being repeatedly introduced to communities from an outside source, likely an animal. One such study published in Science this summer concluded that so far Ebola circulating in Sierra Leone does not appear to have originated from multiple reintroductions of the virus. Rather, by sequencing 99 Ebola virus genome sequences of the majority of Ebola patients in Sierra Leone this past spring the group found that all the cases were traceable to a “patient zero” of Ebola in the community. Yet if there was continuous reintroduction of the same strain of the virus from animals to humans there may not be significant enough mutations to detect what was happening and it could appear to be a continuous chain of transmission, cautions Gary Kobinger, head of the special pathogens program at the Public Health Agency of Canada. And if the virus, hypothetically, somehow adapted to the human population and became less aggressive over time then perhaps that would provide an early sign of endemicity, he says. But tracing that evolution would prove quite challenging. Still, top Ebola experts are not ready to start calling Ebola endemic, at least not yet.



One thing is certain: If Ebola is still persisting a year from now, “The whole response will need to be integrated back into the health system,” says Dye. Although changes to the Ebola response – like creating isolation units at hospitals in west Africa – are under active discussion, no plans are being made right now because the focus still needs to be on the emergency response, he says. Yet if Ebola does become truly endemic – perpetuating itself through the human population – that’s what would be needed. For starters, local health infrastructure would have to be significantly shored up to face such a harsh and long-standing threat. And health officials would need to be ready to immediately transport Ebola patients from one part of the country to isolation wards in another part of the country, says Dye.



The end result would need to look much more like the health care system in the U.S. or western Europe. In those locations dangerous infectious viruses appear relatively rarely and affected patients are placed in special isolation units at hospitals. That setup would be a significant financial and logistical undertaking for African nations, vastly different from the stand-alone specialized Ebola treatment units that currently accommodate hundreds of patients at a time. Gregory Taylor, the chief public health officer of Canada, states that the public health infrastructure build-up would also mean expanding west African lab capacities, something Canada has already been assisting with. And if low levels of Ebola manage to persist throughout 2015, Dye says, such infrastructure actions will need to be taken. After all, underestimating the power of Ebola to spread across west Africa is how the virus was able to flourish in the first place.

