Evil tundra fort, 89 units strong, although 20 of which were children that all seemingly showed up within the first year. Indeed, I unpaused the game and got two weddings within the month! It was going relatively well, excluding an unfortunate incident with the children in the mines when attempting to breach a magma tube... but that's not what brought down the fort. The child mining accident was a harbinger of disaster to come.
One autumn, the merchants of the mountain home decided that the best way into my fort would be to path directly through a horde of bear man thralls. You see, the evil biome randomly spawns evil fogs and devilish ash weather that causes an incurable "opposed to all life" condition although it's unclear to me whether the afflicted were truly undead. I had really been meaning to do something about them, but I had been busy properly expanding the military with bronze. And so, I closed the gates to protect my young and inexperienced fighters from the conflict/shield their eyes to prevent the blood lust that would cause them to charge into battle and also their deaths.
The merchants fought valiantly, and one by one they fell in glorious battle. In time, the bear men wandered off the map, and so the coast was clear for some time. I eventually decided to open the gates and allow collection of those spoils that did not interest the bear men. My busy fort was in a rapid expansion phase. The children were mostly the only ones idle, and so about two dozen children wandered out into the frozen wasteland to collect the wares.
One of those children had a cat.
The ground was spotted with devilish ash.
Cats don't wear shoes.
As the group returned to the fort, the cat suddenly went wild presumably after grooming itself and attacked the nearby children. The fighting appeared to spread the illness extremely quickly. Within the day, half the fort was dead or dying. There was no hope to mount a response of any kind. Anyone who entered the fray became enthralled themselves.
In the future, with syndrome-bearing contamination, I think it would be best to keep the pets locked away.