The Black Death came to Europe in the 14th century, probably mostly aboard merchant ships from the Crimean peninsula in the Black Sea, though soldiers returning from late, intermittent Crusade-type forays likely contributed to the pandemic as well.
In 1346, a Tartar army laid seige to Caffa, a port in Crimea, now an autonomous republic just south of Ukraine. Caffa, now called Feodosija, was then held by Genoa, a mighty Italian city-state and commercial power. The seige lasted three years, and the residents and refugees in Caffa were in a bad way by the end. But then the Tartars began to die in great numbers from a horrific disease, and the tide appeared to be turning.
The remaining Tartars got the idea of catapulting the rotting corpses of their plague victims into the walled city of Caffa. Apparently, the Tartars' hope was that the "intolerable stench would kill everyone inside," according to an account by the Italian notary and writer Gabriele de’ Mussi. At the time, "miasmas," or noxious airs from rotting organic matter, were thought to be one source of disease.
Soon, even though the cadavers were dumped into the sea, people began to die in the besieged city. And then, the survivors began to flee.
De' Mussi writes:
Among those who escaped from Caffa by boat were a few sailors who had been infected with the poisonous disease. Some boats were bound for Genoa, others went to Venice and to other Christian areas. When the sailors reached these places and mixed with the people there, it was as if they had brought evil spirits with them: every city, every settlement, every place was poisoned by the contagious pestilence, and their inhabitants, both men and women, died suddenly. And when one person had contracted the illness, he poisoned his whole family even as he fell and died, so that those preparing to bury his body were seized by death in the same way.
De' Mussi was not an eye-witness to the events in Caffa. However, his account preserved not only the details of what has been remarked upon as an early instance of biological warfare, but also what the survivors learned about how diseases can be transmitted.