The Black Death struck Europe in 1347, killing 30-50% of the European population in six violent years. It wasn’t a one-off epidemic: it signalled the start of the second plague pandemic in Europe that lasted for hundreds of years and only slowly disappeared from the continent after the Great Plague of London in 1665-1666.

These outbreaks were traditionally thought to be caused by rodent reservoirs of infected rats lurking in Europe’s cities, or potentially by rodent reservoirs in the wilderness. But our research, published in the journal PNAS, suggests otherwise.

If the “reservoir” thesis were correct, we would expect plague outbreaks to be associated with local climate fluctuations, through changes in agricultural yields and primary productions in forests, affecting the number of urban and wildlife rodents, resulting in more plague. We found that Europe’s plague outbreaks were indeed associated with climate fluctuations – but in Asia.