Please have a look at a collateral result from climate historical research published on the Mittelalterblog: You start looking for bad weather 700 years ago, and you end up with indications of a flu pandemic…
Martin Bauch, Under the weather: Medieval diseases with low mortality and high contagion, in: Mittelalter. Interdisziplinäre Forschung und Rezeptionsgeschichte, 9. März 2020, https://mittelalter.hypotheses.org/25087