What was the Vad Velen (Yellow Pestilence)?

Did the Yellow Plague cause 6th century Britons to leave for Brittany?

Irish invaders, the plague and the bloody Saxons. All of these are suggested causes of the 6th century migration of Britons from Wales, Cornwall, Devon and Dorset in the sixth century to Brittany. 

The Saxons are now viewed as an unlikely immediate cause – their settlements were too far east. The Irish were certainly a contributory factor. But the plague – what was all that about?

Saint Brioc, on a visit to Cardigan (Ceredigion, Wales) in 526 A.D., wrote that the whole area had been devastated by the plague. The epidemic in question was called y vad velen (‘the yellow pestilence’) and outbreaks of it occurred in Britain in 526, 537 and 547 C.E.

Vad velen is sometimes identified with the ‘Great Plague of Justinian’ of  542 C.E. which arrived in Britain in 547. But Procopius (in 550) called it a bubonic plague - Welsh texts insist on a ‘Yellow Plague’. 
Saint Teilo said it was 'called yellow, because it makes everyone it attacks yellow and bloodless.' He described its arrival in a ‘column of watery cloud’. Many accounts record saints, missionaries and princes fleeing as it spread across ancient Britain.

In 547-548 C.E. Maelgwn, one of the most powerful of Welsh kings, fell victim to the vad velen. It was said that he took refuge in the church at Llanrhos, a holy sanctuary where he would be safe from the evil disease. Curiosity, however, got the better of him and he was tempted to look out through the keyhole of the door. He caught the plague and died.

It is said that his tyranny (and heaps of his unburied victims) brought about his downfall:

"A most strange creature will come from the sea marsh of Rhianedd,

As a punishment of iniquity on Maelgwn Gwynedd;

His hair and his teeth and his eyes being as gold;

And this will bring destruction On Maelgwn Gwynedd."

So what was y vad velen?

Perhaps it wasn’t a plague at all but ‘relapsing fever’. Relapsing fever often appears at the same time as plague or famine. The Great Hunger in Ireland (Irish Famine) was associated with it:

"In Ireland fiabhras buidhe (‘yellow fever’), cut down a lesser but significant swathe than famine fever or typhus. It was a puzzling disease, which doctors called relapsing fever and it was carried by lice ... A violent fever struck the victim for perhaps five days, when the patient recovered and was able to walk about...Perhaps a week later, the illness struck again, in very sudden and increased intensity, often jaundicing the victim (hence 'yellow fever') before felling him."  [Thomas Keneally, 'Three Famines']

Louse-borne relapsing fever occurs today in the developing world wherever there are poor living conditions, war and famine (recently there have been outbreaks in Ethiopia and the Sudan). Symptoms include severe jaundice and chronic bleeding - both of which match Saint Teilo's description of y vad velen perfectly.

Maybe we have found another contributory cause for the migration to Brittany… and the death of the tyrant of Gwynedd.