The bubonic plague, or Black Death, which killed about 50% of Europe’s population in the mid-1300s, was one of the worst pandemics ever known. About 2,000 years ago black rats, which lived around farms, caught bubonic plague from other species of rats that were infested with plague-carrying fleas. Fleas carried the bacteria that caused the plague and transferred it to the rats. Rats were rampant during this time period. When the fleas found a warm-blooded animal, they jumped onto it, drank its blood, and transmitted the plague. When infected rats died, the fleas hopped off them and onto other rats—or nearby humans. Bubonic plague was usually fatal to any person who became infected.