Did you know

That the period between 1550 and 1850 was called the little ice age and that the onset of bubonic plague epidemic in Europe approximately coincided with the little ice age, even though the BP, as a pathogen has been in existence for a long time then?

Apparently the fleas that spread BP to humans only do so below a certain temperature threshold. Above that threshold the fleas can continue to feed off rats with no trouble. Below a certain temperature, the plague pathogen causes clotting in the gut of the flea, causing it to slowly starve to death no matter how much it tries to feed. In this situation the flea is more likely to feed off any animal (rather than their usual target species) out of sheer desperation. Healthy fleas usually
prefer feeding on only one specific host species.

So the lower the temperature gets, the more likely infected fleas are to try
feeding on human blood (or so the theory goes).

Source: DML
There is much more to the world than we think we know.