Warning: Kind of disgusting... Sorry about that.
The Black Death was an extremely deadly disease that infected people nearly 1500 years ago in 1348. It was a bubonic plague, which means that it gave the victims large bumps on their skin that sometimes would pop and ooze pus and blood*. Luckily, The Black Death can no longer be found and will not infect anybody anytime soon. But back then, people would try to avoid getting the disease by breathing herbal leaves and pomander. The worst idea was probably soaking themselves in sewer water*, because the Black Death was coming from diseased animals that were getting into their drinking water.
Oh, and fun fact here: The nursery rhyme "Ring Around the Roses" is actually about the plague. People without the Black Death would put lavender around their children's beds, and the line "ashes ashes, we all fall down" is a nice way to say that people burned the victims that got the Black Death. Whoops.
The Black Death spread all across Europe (Except a place called Cracow, as well as Milan.) in just seven years. As mentioned above, this plague is caused by diseased animals that get into people's water. Now for the science. A species of flea call the rat flea that are infected with a certain type of bacteria called Yersinia pestis. This bacteria travels through the animal, and it dies from being intoxicated, but the creature is still inside the wells that people will drink from. Y. Pestis will then get into the water, and everyone who drinks that water will get the Black Death, then they will give their disease to another person, and then there's a huge chain, spreading rapidly.
There is an estimated death toll that is around 75-200 million people, as told by historian Philip Daileader. Even though this disease was incredibly awful, we can learn from it. Obviously, we don't
want to get anything similar to it, so we can remember to do things to avoid anything like this. No dirty water. Bad, bad dirty water.
(Also, I don't really want to include pictures...)