In 1348, a new disease reached England, having spread from the East. It was unfamiliar to all, and killed rich and poor alike.
They didn’t know this at the time but the plague arrived in England on rats carried by ships and was spread by flea bites.
Once caught, the person died in a few days. In London, it killed 200 people a day. Historians estimate that 1/3 of the population died as a result of this epidemic.