Luckily, I'm a biologist/biochemist
The "Black Plague", Black Death, or Bulbonic Plague, is transmitted from rats, to fleas, and back & forth... then to Humans via the fleas. Fleas are the carrier. Without a rat infestation, the disease never really gets a foothold. Without fleas to carry it, it never gets transmittted. The bacteria is
Yersinia pestis
Bad sanitation in the middile ages allowed it to manifest via a rat infestation first, then all the fleas that came along with the rats. It quickly passed to Humans where it was transmitted by aerosol & contact - much like influenza (cold & flu). It's mostly inactive in fleas because their body temperature is too low (cold-blooded). The bacterium is then "active" in humans with a warm enough body temperature to support it's full growth & reproduction. (and rats & other mammals).
Couple days, maybe a week, flu-like symptoms: fever or chills, headaches, aches, etc. The lymph nodes can swell to the size of golf balls, or much larger even (from their usual pea size). The bacterium multiplies and fills the blood stream, clogging it, choking blood supply and causing massive internal & external hemorraging, and grotesque, necrotic skin lesions. It fills, clogs, and ruptures the lungs as well, giving pneumonia-like symptoms.
In contrast to the usual horror stories you hear about it, the mortality rate is only about 30%. "Only" ...being still over 30 million people dead in Europe at the time. With good treatment and early enough detection, nearly everyone recovers fully, though, nowadays.
That said... that is the wild/natural form of the bacteria. Much, much... much more infectious and deadly forms were produced in biological weapons labs around the world...
...wild animal populations with Plague still exist today... worldwide except in Europe & Australia/NZ, and inside the artic circle (alaska, N Canda, N Siberia).
