In the middle of the fourteenth century, Cork City, already in economic decline and harried by the rebellious Gaelic population outside the walls, was devastated by one of the greatest calamities ever to befall Europe. This was the outbreak of the plague, which came to be known as the Black Death.
The plague is caused by a bacterium called Yersinia pestis, which is carried by fleas on the black rat.
European merchant ships trading with the Crimea brought the black rat to Europe and the first recorded outbreak of the plague in Europe was in Sicily in 1347. It had reached Dalkey by August 1348 and seems to have reached Cork sometime in the summer of 1349.
The plague comes in two forms, bubonic and pneumonic. Bubonic plague is spread by a bite from a flea; pneumonic plague, which is far more deadly, is spread directly from human to human. Both forms are recorded in Ireland but pneumonic plague appears to have been more common. The housing conditions and poor sanitation in Cork and other European cities helped the spread of the disease, and medieval Ireland was notorious for the prevalence of fleas.
Very little contemporary evidence has survived on the plague in Cork, but, extrapolating from contemporary evidence from other Irish and European sources, it is likely that between 25% and 35% of the population of the city died. At that time the population is estimated to have been about 2,000. It had a devastating effect on the social and economic life of the city and the governing council of the city appealed for remittances and reductions in the fee farm payable annually to the Crown. The Black Death had relatively little impact on the Gaelic Irish in the rural areas, which further tilted the balance of power against the city.
The plague became endemic in medieval Ireland and many other outbreaks are recorded in the later fourteenth century. None of them were as devastating as the Black Death itself.