In the summer of 541 AD a deadly infectious disease broke out in the Egyptian port city of Pelusium, located on the eastern edge of the Nile delta. It quickly spread eastward along the coast to Gaza and westward to Alexandria. By the following spring it had found its way to Constantinople, capital of the Roman Empire. Syria, Anatolia, Greece, Italy, Gaul, Iberia, and North Africa: none of the lands bordering the Mediterranean escaped it. Here and there, it followed river valleys or overland routes and thus penetrated far into the interior, reaching, for example, as far east as Persia or as far north, after another sea-crossing, as the British Isles.

The disease remained virulent in these lands for slighty more than two centuries, although it never settled anywhere for long. Instead, it came and went, and as is frequently the case with unwelcome visitors, its appearances were unannounced. Overall, there was not a decade in the course of those two centuries when it was not inflicting death somewhere in the Mediterranean region. In those places where it appeared several times, the intervals between recurrences ranged from about six to twenty years. And then, in the middle of the eighth century, it vanished with as little ceremony as when it first arrived.