The Black Death is generally understood to have been caused by the flea on a rat that appeared in Europe from Asia, having come from the steppes.  The Black Death came in three forms:  bubonic, pneumonic, and septicemic, all caused by a bacterium called Yersinia pestis.    These three forms had a mortality rate of 30-75%, 90-95%, and 100% respectively.

http://www.insecta-inspecta.com/fleas/bdeath/Black.html

Skip Knox writes:  ‘The Black Death erupted in the Gobi Desert in the late 1320s. No one really knows why. The plague bacillus was alive and active long before that; indeed Europe itself had suffered an epidemic in the 6th century. But the disease had lain relatively dormant in the succeeding centuries. We know that the climate of Earth began to cool in the 14th century, and perhaps this so-called little Ice Age had something to do with it.  Whatever the reason, we know that the outbreak began there and spread outward. While it did go west, it spread in every direction, and the Asian nations suffered as cruelly as anywhere. In China, for example, the population dropped from around 125 million to 90 million over the course of the 14thc.’

While the Black Death arrived in Italy in 1347, it didn’t reach Wales until early 1348 or 1349, probably carried from southern England.  In general, the best guess is that the plague killed 1/3 the population of Europe, and there is no reason to think Wales was any different, except that it was more rural, still, than much of Europe.  The weather had grown colder in the last 100 years, however, and with the pressure of the English conquest and the subsequent altered social makeup of Wales, more displacement and death may have occurred there than in other places.

The following is a map of the spread of the plague:  http://www.insecta-inspecta.com/fleas/bdeath/Path.html 

Mike Ibeji writes:  ‘The plague in Wales and the Marches were as pitiless as elsewhere. At Whitchurch, an inquest into the death of one John le Strange revealed that John had died on 20th August 1349. His oldest son, Fulk, died 2 days before the inquest could be held on 30th August. Before an inquest could be held on Fulk’s estate, his brother Humphrey was dead too. John, the third brother, survived to inherit a shattered estate, in which the 3 water mills which belonged to him were assessed at only half their value ‘by reason of the want of those grinding, on account of the pestilence.’ His land was deemed worthless because all its tenants were dead ‘and no-one is willing to hire the land.’

Jean Geuthin, a Welsh poet who himself was dead by 1349 wrote at the time:  ‘We see death coming into our midst like black smoke, a plague which cuts off the young, a rootless phantom which has no mercy or fair countenance. Woe is me of the shilling in the arm-pit; it is seething, terrible, wherever it may come, a head that gives pain and causes a loud cry, a burden carried under the arms, a painful angry knob, a white lump. It is of the form of an apple, like the head of an onion, a small boil that spares no-one. Great is its seething, like a burning cinder, a grievous thing of an ashy colour. It is an ugly eruption that comes with unseemly haste. It is a grievous ornament that breaks out in a rash. The early ornaments of black death.’