A huge problem I feel is that communication between the several scientific fields implicated in the research on the Black Death is rickety at best.

I've listened to a virology podcast recently that spoke about Yersinia Pestis and how it propagates and they know and have known for a while that rats are just one of the vectors of the Plague. They got a lot of their History wrong though, which is really funny. Apparently, we focused way too much on rats and the bubonic, pulmonary and septicemic stages of the disease.

Now, I will try to vulgarize it, be gentle, please!

The initial propagation happens when a flea bites an infected rodent (any rodent, this is important) and then bites a human, which infects him with the bacteria. We'll call this human patient zero, P0. The flea continues its nasty job of biting and infecting humans and rodents until it dies of hunger, apparently.

P0 develops the symptoms and begans to secrete infected pus from the buboes that grows on his body. His saliva and blood also contains a lot of bacteria. So, P0 cough, bleeds and "pusses" all over the place, and then infects other humans. This is when the plague becomes an epidemic.

Now, some rodents are sporadic (once in a while, a colony becomes infected) carriers of the bacteria: marmots principally, rabbits, rats too. Steppe marmots were one of the staple food of Mongols and other Central Asian nomads. They carried them all over the place. At some point, some Mongols carried infected marmots out of Mongolia and due to unique circumstances, including the speed of the Mongol armies and post system, carried either infected rodents or an infected P0 and the Plague became a pandemic.

We focus a lot on the siege of Caffa in 1344 because it's when the first cases are known to Western sources. But evidence shows it was devastating in the Middle East, North Africa and China too.

As for the spread of the disease, an Italian galley could move from Crimea to Genoa in less than two weeks. Another galley coming out of Genoa could spread the "good news" to London in another month, stopping in several ports in Italy, Spain, Portugal and France along the way. By then, you have dozen of infectious hotspots and half of Europe's population would die in the next 7 years.