I hate it when this happens: Carbon dioxide emitted in Topeka causes rain in dry areas of China, which causes more vegetation, which causes more rats, which causes more fleas, which causes more Black Death

Climate Change May Worsen Plague | LiveScience

They found that in the northern regions of China, which generally has a dry climate, increased rainfall was linked to more cases of plague; the researchers suspect the wetter conditions gave rise to more vegetation, so flea-bearing rodents had more food. More fleas that can carry the Y. pestis pathogen would mean more plague cases.

The Little Ice Age in Europe

Malnutrition led to a weakened immunity to a variety of illnesses. In England, malnutrition aggravated an influenza epidemic of 1557-8 in which whole families died. In fact, during most of the 1550's deaths outnumbered births (Lamb, 1995.) The Black Death (Bubonic Plague) was hastened by malnutrition all over Europe.