Warfare from centuries ago was certainly different to how we experience it now. It seems that there have also been times where individuals have been more than willing to use anything that they could get their hands on in order to try to attack their enemy, but this time it could be argued that things just went a bit too far.
In 1347, the Tartars were laying siege to the city of Kaffa, and the inhabitants were understandably becoming rather fed up with the entire scenario. It was also a time when the plague was across the entire area, but it seems that the Tartars had a different idea as to how to effectively take control of the city.
As the city itself had been cut off, it meant that the plague was not such a big deal as it was in other parts of Europe. The Tartars were also open to trying anything that they could to win, so they then came up with the idea to use the bodies of those that had died from the plague as missiles.
In doing so, they basically threw the bodies into the city, which was a blatant attempt to try to get the plague into the confines of the city knowing that people were effectively living on top of one another. It was something that would ultimately work, as people in the city did eventually die from the plague, and in the process of doing so it reduced the level of resistance against the Tartars.