Rats, of course, have fleas (Xenopsylla Cheopis I believe) which feed on the rat’s
blood and digest it with the aid of an enzyme. Like most fleas, they tend to be species specific, and stick to the host they know.
However, below a certain temperature this enzyme
ceases to function. So the flea, despite having a
belly full of blood is unable to derive any sustenance
from it and remains as hungry as ever. A hungry flea,
is an angry flea and will, as like as not, depart its
rat host and proceed to bite anything vaguely
mammalian in the vicinity (they can smell exhaled carbon dioxide apparently), in a vain attempt to find a source of fresh blood that will satisfy its voracious appetite.
Unfortunately these fleas themselves act as host (or at least a vector) for a number of interesting micro-organisms including the Yersinia pestis bacillum that causes the bubonic plague in humans. And hence a period of global cooling such as that created by the disaster of 535 will trigger off a plague epidemic.
Or so the theory goes. Based on my recollection from some TV documentary some years ago. Part of a series called Secrets of the Dead or something like that (essentially advancing the Krakatoa 535 theory). Attempts of mine to provide some sort of confirmation for this bit of flea biochemistry has proved elusive. But I will note that between the plague epidemics of the 6th century and those of the 14th (see the Black Death) there was the climatological phenomenon called the Medieval Warm Period otherwise known as the Medieval Optimum.