Macha was the fairy wife of Crundchu, a wealthy Ulster farmer, who she could only stay with as long as he told no-one about her. Unfortunately at the Great Fair of the Ultonians, he forgot her Injunction, and mentioned her running ability to the Ultonian King. The King ordered her brought before him and demanded that she prove her swiftness by racing his horses. She was heavily pregnant, and begged leave to seclude herself to give birth before the race. The Ultonians refused and the poor pregnant woman was forced to run aginst the King's horses. She beat them, and gave birth on the finishing line to twins. As she screamed in agony, the Ultonians were likewise gripped by labour pains, and lay helpless. And Macha prophesied : "From this hour the shame you have wrought on me will fall upon each man of Ulster. In the hours of your greatest need ye shall be weak and helpless as women in childbirth, and this shall endure for five days and four nights - to the ninth generation the curse shall be upon you."
And so it was that the Debility of the Ultonians afflicted the warriors of the province periodically.
The events of this legend are reckoned to have taken place over two thousand years ago, but the Debility seems to describe some kind of fatigue syndrome, where the sufferers, all men of Ulster, lie in their beds, moaning and writhing, unable to lift a spear. What's interesting, from a medical viewpoint is whether these legends are actually describing a real condition - maybe chronic fatique syndrome, post-viral fatigue syndrome, or even myalgic encephalomyelitis?
It would be an interesting exercise in forensic pathogenicarchaeology, to test the present day descendents of the Ultonians, to see if their immune systems carry any antibodies to these syndromes, or whether they are more prone, or more resistant to them.