The study authors believe an eruption occurred around 1345, about two years before the start of the pandemic, from either a single volcano or a cluster of volcanoes of unknown location, likely in the tropics. The resulting haze from volcanic ash would have partially blocked sunlight across the Mediterranean region over multiple years, causing temperatures to drop and crops to fail.

An ensuing grain shortage threatened to spark a famine or civil unrest, so Italian city-states, such as Venice and Genoa, resorted to emergency imports from the Black Sea region, which helped keep the population fed.

However, ships that carried the grain were loaded with a deadly bacterium: Yersinia pestis. The pathogen, originating from wild rodent populations in Central Asia, went on to cause the plague that devastated Europe.