Oops! An accident killed off one third of the world.
A germ warfare attack got out-of-control and grew into the Black Plague. Within three years, one third of the known world's population had perished.
The 1347 Tartar siege of the city of Kaffa started it. Kaffa was an Italian trading port on the Crimean Peninsula in the Black Sea. Their host Tartars alternately tolerated and proscribed its commerce, depending on who was Sultan (and, no doubt, on the size of the tributes paid.) After recent Tartar problems, the Italians fortified their town with two walls. Those held the Tartar enemy at bay in an attack of 1343. An Italian army vanquished the besieging Tartars.
A large Tartar army returned in 1345 and laid siege to Kaffa again. An army is more than a group of soldiers. Caravans of personnel, equipment and supplies to maintain the army are part of its entourage. Vermin hitchhike on any such large-scale project. Rats, mice and bugs are unavoidable companions.
Some of the rats following this expedition hosted fleas carrying the Bubonic Plague germ. They had come from rural areas where there were few people, so they had primarily infected the rats. The crowding of an army at siege offered a supply of human targets. Human infection followed. No surprise: losses to disease were expected.
The disease spread quickly through the Tartar ranks. Bodies piled up. The danger of dead bodies was well known; they spread the disease. Those dead bodies were viable biological weapons. Hurl them over the walls and infect the population within. The Tartars had plenty of bodies to throw. Try for the water supply, and hope for the worst. Those cleaning up and disposing of the remains picked up their fleas, and the plague entered the city.
Under siege with a deadly disease running rampant, Kaffa was Hell on Earth. The city was a port. Rich merchants were able to escape by ship. They crossed the Black Sea to Constantinople. The plague broke out there. Their ships' rats had brought the disease with them. The merchants continued on home to Italy. The plague followed them there.
The plague followed the trade routes west to Spain and then north to involve most of Europe. In three years, it had reached Scandinavia. Bodies piled up. One in three died and no one knew why. The Jews were merchants. They followed those trade routes. This must be part of a Jewish pact with the devil to wipe out Christianity. Certainly one more good reason to persecute the Jews … and persecute them they did. Depopulation changed European society forever.
Meanwhile, back at Kaffa, the plague had decimated the Tartar army. The remaining troops could no longer sustain the siege. They broke it off and returned home-carrying the plague back to most of Asia.
In three years time, one third of the population of Europe, Asia and North Africa had died from a germ warfare attack that had gotten out-of-control. It could happen again. Germs are hard to control, and quarantines are hard to enforce.