The mother of all pandemics

The recent covid-19 global pandemic is  causing serious problems worldwide, but it's nothing compared to the Black Death of the Middle Ages.

The plague swept Eurasia, North Africa and Europe killing up to 200 million people in just 4 years. It's known it was caused by a highly infectious bacteria, and the popular idea of how it spread was via the fleas on rats. Samples from plague victims graves suggest this is the case, since the fleas are still present on the bodies.

However, it's the rats which getting the blame for something they didn't do. In particular, some studies have shown the rate of spread couldn't have been as fast as it was if it were the rats alone. This is because most rats live their entire life around 100 feet from where they were born. They are not natural migratory animals. Also, the sheer number of rats required to cover the areas infected meant there must have been large numbers of dead ones literally everywhere, and there are no reports of these either.

What is known, however, is that the humans of the day had incredibly poor hygiene... 

It's still the fleas

Back in the middle ages, everyone's hygiene was very different to that of today. It's not just the lack of fresh water and all the other modern facilities we all take for granted that was the problem. It was more of an attitude and awareness issue. Put simpley, people didn't think it was important to keep clean. This is largely down to education. If they realised that most of their everyday helth issues were caused by the lack of cleanlimess, they would have tried to do something about it, but were in fact totally ignorant of these ideas. The vast majority of the human race could not read or write, and even if they could there was nothing printed to help them.

It is certain the plague was spead by fleas, but those fleas would most likely have been on the humans themselves. This would explain the speed of its spread, and the notable lack of dead rats required, which would have somehow suddenly gone against their non-migratory instincts collectively as was throught previously.