Answer:
The Black Death
Step-by-step explanation:
The Black Death bascially destroyed Europe, being a fatal disease. The reason why it was so deadly though was because of how fast it spread. Animals like rats first got the disease, then it spread to humans. The diease spread so quickly was because back then the towns were dirty and crowded, which rats often lived in, which caused rats to make common contact with humans.
Hope this helps! :)