Years ago Ebola made itself known to scientists, when it appeared simultaneously in the Sudan and Zaire. The two events were a very long way from each other. It happens that I am very familiar with that part of the map, and I’m certain that any attempt to go from Nzara, Sudan, to Yambuku Zaire on land would take several weeks and, actually, be impossible. It could not happen casually. For a while, experts thought a particular person who was probably patient zero at Yambuku had made the trip, despite no evidence for him having done so. In the end, most ebola experts simply stopped thinking about this conundrum. A few of us working in the area, though, had a different idea. Animal-born (we thought fruit bat) ebola spread in the animal first, and conditions emerged that heightened the chance of a jump to humans also spread, so there were two separate jumps. Likely, this could happen now and then, with several jumps within a few weeks time, but only during those few weeks time when conditions were just right. The trick to managing future ebola outbreaks might be to figure out what those conditions might be, and at least, set up a warning system. But, since epidemiology worked at the time entirely on the pump model, one source, one initial spread, that sort of thinking never happened.

If that is typical for zoonotic diseases (even if not inevitable in every case) it presents a slightly different view than what one usually conjures up. It is not the case that an animal sneezes or bleeds (or whatever) on a human, then that humna, patient zero spreads the disease to other humans. Rather, the condition of transfer from an animal reservoir becomes temporarily highly likely insead of almost impossible, and perhaps dozens of transfers happen, of which, one or two or three, perhaps, are traced to eventually by epidemiologists.

Turns out that is what probably happened with Covid-19. The transfer happened twice, over just a few weeks time. The best explanation for this is that some animal species (could be more than one) had their own epidemic of this particular coronavirus strain going on, and there happen to be a big market with this animal species (or species) on sale, and the rest is history.

There are two studies, this one and this one, seem to support this idea. When the disease experts are done being incredibly busy with Covid, maybe they can go back to Sudan/Congo and rethink the initial appearance of Ebola with this model, now no longer just some zany idea a few of us had years ago, in mind.