Infected at Caffa

It is commonly believed that the Black Death entered Europe during the Siege of Caffa. The invading Mongol army had been infected with the disease. Realizing they didn't have enough manpower to win, the Mongols were forced to retreat. But the vengeful generals then did something that would change the course of history. They decided not to give the citizens a peaceful victory and flung the bodies of infected soldiers into the city. 

Across the Continent

The horrified citizens fled the city in droves. They took ships straight out of the city and sailed into other ports. What they didn't realize until too late what that they brought the disease with them. Stowed away on their bodies were the carrier fleas which would infect not only them, but also hitch a ride on the rats. The rats would leave and the disease would infect just about everyone. Unknown to them, this would be the first ever instance of a biological war.