Black Death (oversimplified)

Messinian Sailors and trades got here, got infected and went back to Europe. By the time they realized, it was too late.
In 1333, the province of Hopei in china, a random epidemic appeared, killing 5 million Chinese people. Hopei was a major trading city, big problem.
The plague ravaged England. It killed an estimated 40% of the population. In just 18 months, it killed 400k people, nothing experienced before
Poor hygiene and cramped housing worsened the plague, Y. Pestis was the pathogen responsible for this, it lived in fleas which lived in rats, which went all around london
Haha yes, die trash
At around the same time, traders got attacked by turkish tribes, ( known as the Tatars ). Plague victims were piled in the city, which infected more people. By January 1348, some sailors and passengers thought that they escaped the plague.
As the plague was noticed throughout Europe, it was known as the great plague, The river thames was a hub for the disease.
Cemeteries were opened left and right, being officialized by Edward III. As this happened the plague continued to spread.
Another major factor in the spread of the disease was the livestock living with civillians, discarded bodies crammed the streets, then livestock ate waste and came into contact with people