“The music is not in the notes, but the silence in between.”   – Wolfgang Amadeus Mozart

If one were to describe the history of Ebola outbreaks, one method would be to construct a timeline, with a point on the line for each outbreak. You could create this timeline with a varying number of points, depending on your methodology, but regardless of how you built your timeline, there would be spaces between these points. This is due to the nature of Ebola; it appears, it disappears, and it appears again. To the Ebola virus, these gaps are periods of convalescence. To us, they are periods of absence and mystery, and one of these gaps stands out as the most mysterious.

The CDC lists five Ebola outbreaks in the late 1970’s. The “first” Ebola outbreak, according to WHO, took place in 1976, though we now recognize the event as two simultaneous and separate outbreaks. Between June and November 1976, 284 cases (151 deaths) of Ebola Sudan occurred near what is now Nzara, South Sudan; between September and October 1976, 318 cases (280 deaths) of Ebola Zaire occurred near what is now Yambuku, Democratic Republic of Congo (DRC). BMJ reported that in November 1976, a researcher in England that was working with samples from the Nzara outbreak accidentally infected himself; CDC lists this accident as the third Ebola outbreak (the individual recovered). In June 1977, a child became sick and died from Ebola Zaire in Tandala, DRC as found by Heymann, Weisfeld, Webb, et al in the Journal of Infectious Diseases; though there was only one confirmed case, subsequent epidemiological investigations of the area uncovered several other historical, probable cases. Finally, WHO reported that between July and October 1979, 34 cases (22 deaths) of Ebola Sudan occurred, unbelievably, in Nzara, Sudan — the same community where the first cases of Ebola emerged just three years prior. In the span of just 39 months, the terror of Ebola had introduced itself to the world five times (638 cases, 454 deaths) and then… silence.

Ebola would not reappear for ten whole years, and even then, the subtype was Ebola Reston, which does not affect humans. Ebola Reston was first discovered in imported non-human primates designated for laboratory testing. Though it was discovered that humans could be infected with Ebola Reston, the infections were asymptomatic. Though CDC lists four Ebola Reston outbreaks between 1989 and 1992, the world would not see another case of Ebola virus disease in humans until late-1994, in Gabon. Even then, the outbreak (52 cases, 31 deaths) was mischaracterized as yellow fever for several months. Perhaps the virus’s long absence from the spotlight had removed it from the collective consciousness in 1994, certainly in the presence of those pathogens that had been circulating and consuming our attention in the meantime.

The fifteen-year disappearance of Ebola, particularly in light of its frequent and severe outbreaks in the late 1970’s, has perplexed researchers for decades. The mystery lies, to some extent, within the lack of complete knowledge of the virus reservoir, though scientists have long suspected bats. It’s hard to detect disease when you cannot pinpoint the source. Surveillance and reporting has been another confounding element. How many times in that fifteen-year period was an illness misdiagnosed as yellow fever, dengue hemorrhagic fever, or some other similar illness, because of lack of knowledge or diagnostic capabilities, or simply because there was no health care around?  We will probably never be able to answer this question. Finally, our perceived zone of endemicity at the time was limited to northern DRC and southern Sudan. Was the virus appearing elsewhere, unbeknownst to us? We certainly weren’t expecting it to emerge in Gabon in 1994 (and Uganda in 2000, and West Africa in 2014).

Scientists today continue to be perplexed by the emergence of the virus. What brings Ebola out from its hiding place? Is its emergence/re-emergence tied to climate change? globalization? the changing interface between humans and wildlife? If it has to do with any of these increasingly significant factors, how do they explain the fifteen-year disappearance?

These days, the virus comes and goes with some predictability—since 2000, outbreaks have approached a near-annual incidence, sometimes skipping a year, sometimes lasting more than a year. The periods between outbreaks are growing shorter. Is this because our capability to detect Ebola outbreaks is improving, or is the virus able to infect humans more frequently? One thing is for sure: the world knows that when one outbreak ends, another will eventually follow, and we need not wait fifteen years.