The most relevant chain of events that contributed to the Black Death being so deadly is:
Cooling climate led to reduced harvests, which caused people to be ill-nourished and more likely to get sick.
This statement reflects an important historical context where climatic changes, such as the Little Ice Age, resulted in poor agricultural yields. Malnutrition weakened populations and made them more susceptible to diseases like the bubonic plague, exacerbating the impact of the epidemic when it arrived in Europe.