We were playing Carcassonne yesterday, and we started discussing the real-life city of Carcassonne, and the following conversation ensued (loose paraphrase):
Naomi: They had a plague pig! They were under siege, so they infected a pig, and then they threw it over the wall with a trebuchet!
Laura: But wait, how would that work? Are there diseases that can pass from pigs to humans?
Jed: SWINE! FLEW!
I was inordinately gleeful to be handed that opportunity. I was so pleased with myself that I repeated the line a couple more times. I crack me up.
Turns out, alas, that Naomi was conflating two medieval siege stories: first, the story about Dame Carcass and the pig:
Charlemagne lay siege to the town. [As] food was running out one of the town's women came up with an idea [...]. She fed the very last remains of food (probably grain) to the very last pig in the town and then threw it over the wall. Charlemagne's soldiers on the other side of the wall were shocked to think that the town had enough food to throw away fat pigs so, believing that their efforts would never succeed in starving the people out, they ended the siege.
Also described a bit differently on a different page:
The Emperor Charlemagne besieged the town in 795, which was held by Dame Carcass, a Saracen princess. After a five year siege, the only food left was one little pig and a bag of corn. Dame Carcass gave the bag of corn to the pig and sent it out to the ramparts. Charlemagne raised the siege, since he thought there was enough food even to feed a small pig.
Apparently there are stories of diseased cows being lobbed at armies via trebuchet, but I'm having a hard time finding an authoritative source for that via TSOR. Perhaps it's a pre-urban legend.
But I can't resist quoting two snippets that came up when I did that search: