From Live Science:
An unknown volcanic eruption in the mid-14th century may have set the stage for the spread of the Black Death in Europe, according to a new study. By triggering a cool and overcast period in the Mediterranean, the eruption started a domino effect that led to a downturn in agricultural production, which required merchants to import grain — and the bacterium Yersinia pestis that causes bubonic plague — via the Black Sea.
The bubonic plague pandemic, more commonly known as the Black Death, reached Europe in 1347 and quickly affected Italian port cities. The plague then spread throughout Europe over the next few years, resulting in the deaths of between 30% and 60% of the population.