Bubonic plague was one of the most feared diseases of the ancient and medieval worlds.
Nobody could tell what caused it, and most people who got it died within a few days, screaming in pain. And when one or two people in a village got it, usually it spread to everyone else, and many of them died.
Plague caused a fever, and black spots on your chest sometimes, and sometimes great big black swellings on your armpits and at the top of your legs. That's why they called it the Black Death. These swellings got hard like rocks and hurt, and then in a day or two people usually died. There was no effective treatment, though of course people tried all kinds of things, from magic to surgery. Sometimes people did get better on their own, if they had good nursing care and were very healthy to begin with.
Today we do know what causes bubonic plague: it's a bacterium. Fleas carry it in the blood they suck; if a flea bites an infected person or animal (usually it's rats) and then bites you, then you'll get it too. People do still get bubonic plague, even today. But today we can cure it with antibiotics, and so most people who get it live.
DNA evidence shows that the plague came first from China, but the first recorded instance of people getting bubonic plague was in Constantinople about 570 AD. Many people died. Soon afterward the plague spread from Constantinople to Europe. There was another very serious outbreak of plague in 1328 AD which also began in China and by 1347 spread across the Mongol Empire to West Asia, finally killing people in North Africa and in France, England, Germany and Italy. This plague caused the end of the Mongol Empire and killed about one out of every three people in Europe.
Plague epidemics were much worse in places where people were crowded together and didn't get enough to eat. Crowded villages and cities had more bedbugs, lice, and fleas, which bit the rats and then bit people and so spread the disease.