Earth's History

The Black Death spread from central asia along trade routes to southern europe. it swept through the continent and reached Northern Scandinavia and Iceland in 1350. Between 30-40% of the european population by then had perished.
During the cold period now known as the Little Ice Age, (Records from 14th century account of severe weather in 1314 to 1317) we can assume that rodents more then normal take shelter in human homes, resulting in a more effective spread of people inflicted with the bulemic plague,
The decrease in population leads to the 14th Century Climate Instability.

The 1918 influenza pandemic. Unusually deadly influenza pandemic caused by the H1N1 influenza A virus. infected 500 million people in four successive waves. (1/3 of the world population)   Death toll somewhere between 17 million and 100 million, making it one of the deadliest pandemics in (known) human history.