The plague was caused by the bacterium Yersinia pestis and usually spread to humans through fleas on infected rodents. In some cases, it also spread through close contact with infected animals or, for pneumonic plague, through respiratory droplets.

What caused it

  • Bacterial infection: the root cause was Yersinia pestis.
  • Animal reservoir: rodents carried the bacteria.
  • Human infection route: infected fleas bit people, passing the bacteria along.

Why it spread so widely

When rodent populations died off, their fleas often sought new hosts, including humans, which helped outbreaks grow.

In history

The plague caused the Black Death and other major pandemics, making it one of the deadliest diseases in history.

Important note

Today, plague is still treatable with prompt antibiotics, but it can be serious without quick care.

TL;DR: The plague was caused by Yersinia pestis , mainly spread by fleas from infected rodents.