Although Bubonic Plague is normally associated with the Black Death of the middle-ages, it has actually devastated Europe on a number of occasions. One of these outbreaks began in Egypt around the year 540AD. The disease quickly spread across southern Europe, and by the year 550 had struck down the British King Maelgwn.
But, in a curious twist, the plague does not seem to have spread fully across the British Isles. While the Irish and the British Celts (modern Welsh) have folk traditions which speak of this great plague, the English (Anglo-Saxons) have no such record.
The reason for this, according to British Dark Age scholar John Morris* can be determined by looking at the trading partners of the two groups. The Anglo-Saxons did most of their trading with other Germanic tribes from northern Europe where the plague never took hold. Meanwhile, the Celts did much of their trading with southern Europe and Northern Africa, where the plague hit hardest. There is also lots of compelling evidence that the Celts and Anglo-Saxons did their best to avoid one another, and trading between the two groups was almost nonexistent. Thus, once the plague took hold among the British tribes, it never spread to the English.
Although devastating, the plague is not the sole reason that the Anglo-Saxons conquered the Celts, but it was certainly a major factor. In fact, it was only twenty years or so later that the Saxons began their next major push against the Celts, and this time, there would be no Arthur.
· Morris, John. The Age of Arthur: A History of the British Isles from 350 to 650, Phoneix, 2002