Angus Trumble in a delightful piece at the TLS on Ole Benedictow's book about the Black Death, 1346-53:
The best horror stories are real. A flea sinks its proboscis into the skin of a sick black rat, feeds on its blood, and ingests lethal bacteria. In the confined space of its tiny alimentary canal, the bacteria multiply to such an extent that they form a blockage in the stomach of the flea. In desperation, after it senses a drop in the body temperature of the rat, which is by now dead, the increasingly ravenous flea jumps ship. It cannot find another living rat in the nest. Rat nests having for millennia thriven in barns and granaries, the flea does not have to travel far to find an alternative source of food. It searches out the nearest man, woman, or child, maybe burrows its way through layers of clothing, and sinks its proboscis into warm flesh.
The blockage in its stomach prevents the maddened, dying flea from being able to ingest more than a small amount of human blood, and causes it instead to regurgitate tiny amounts of infected rat blood that, breaking free from the blockage, carry thousands of bacteria into the open wound. So gaining entry to the largely defenceless human body, the bacteria travel through the lymphatic system to nodes in the groin or under the arms or in the neck, congregrate, multiply hugely, create a bubo of exquisite, agonizing sensitivity, which duly propels bacteria into the bloodstream that in 80 per cent of cases overwhelm, then kill the sufferer. In rare instances when the flea vomits directly into a tiny blood vessel, the bacteria bypass the lymphatic system entirely, further multiply at a dizzying rate, bringing death in a matter of hours, with 100 per cent mortality. Thus, in a vignette that has been played out by thousands of generations of fleas, Yersinia pestis, the seemingly unstoppable bacterium we know and fear as the bubonic plague, plays leapfrog from rat to person, and runs its ghastly course.
Mmmm . . . amazing.