Interesting question. The black death was a rather ready virus or as they called it a "plague". The reason it was so deadly it because it was a brand new virus/plague that the Europeans had never encounterd therefore being rather deadly for there immune system was not ready for it and had no way of treating it because it was an known virus from overseas. If you didn't know it was carried over on a ship where the virus was attached to a rat with further infected the crew and soon after the entire towb
What was said below is true but also because there was no way of actually treating the disease and stupid cures were invented for it which of course didn't work. They tried getting rid of all animals but getting rid of rats was hard due to the living conditions of medieval times. Considering this we learn that it killed a third of the population of Europe. That's several millions of people. However it didn't only have a devastating effect on Europe, it had it everywhere. While typically we focus on plague in England and Europe it is important to realise that it came from the Middle East or even further away in India and that it had devastating effects there too.
