"I absolutely wasn't looking for that," Allaby said, but "it comes through loud and clear from a bunch of different angles."
He said he found no changes in the archaeological record until "this period of the Crusades. Then I suddenly see barley, I see barley coming from outside, I see the virus arrive, so the narrative more or less writes itself."
The Seventh Crusade ended with the capture of Louis IX in North Africa, but the virus continued to spread.
Unlike many viruses, it cannot migrate on its own, or travel with the wind. A plant with the virus has to physically touch another plant to transmit the disease, so plants must have been carried great distances, probably along trade routes, including the fabled Silk Road to China.
That also fits with Allaby's theory. The crusades lasted two centuries, involving tens of thousands of warriors, so vast lands must have been required to grow food, including barley, and plants must have been transported to distant lands to feed the crusaders.
There is, alas, no answer yet to the question of where and why the virus originated. In fact, the origins of most viruses are unknown.
The Scientific American summed up the current status in these words: "At the end of the day, however, despite all of their common features and unique abilities to copy and spread their genomes, the origins of most viruses may remain forever obscure."
Where they came from, and how they started, disappeared long ago with the RNA.