Please answer a question about the following article about Cork (city):

The city's municipal government was dominated by about 12–15 merchant families, whose wealth came from overseas trade with continental Europe — in particular the export of wool and hides and the import of salt, iron and wine. The medieval population of Cork was about 2,100 people. It suffered a severe blow in 1349 when almost half the townspeople died of plague when the Black Death arrived in the town. In 1491, Cork played a part in the English Wars of the Roses when Perkin Warbeck a pretender to the English throne, landed in the city and tried to recruit support for a plot to overthrow Henry VII of England. The then mayor of Cork and several important citizens went with Warbeck to England but when the rebellion collapsed they were all captured and executed. The title of Mayor of Cork was established by royal charter in 1318, and the title was changed to Lord Mayor in 1900 following the knighthood of the incumbent Mayor by Queen Victoria on her Royal visit to the city.

What caused the deaths of half of Cork's citizens?
Black Death