And, even the fastestvand nastiest of the “Black Death” symptoms seem spreading by secondary infection and tertiary vectors (infected human to infect flea to carry by rat to infect fleas to infect next human) appears too slow.
I suspect something different. Something spread directly person to person like the 1918 flu was spread. Airborne transmitted, yes. But by lingering contact also.
I mostly agree but wasn’t there a pneumonia form of the Black Death that developed that could spread directly from aerosol droplets?
Probably not just one bug, as someone pointed out above, lots of critters arrived. And in the millennia before microbiology, two-way transmission of diseases were just part of saying hello. :^)