I had a successful fort of 80+ dwarves and 29k+ exported wealth, called Townpage. I had a tavern ("The Dipped Olives") and two temples. There were no tantrums, and I had the materials when people got strange moods. Everyone was happy. Then the werepossum showed up. The well-trained military dispatched it in the blink of an eye. But the military commander got injured in the process, and unbeknowest to everyone, got infected.
The small hospital was used for the first time. The chief medical dwarf and various medical staff were able to diagnose and treat most of his injuries. He was released into the public. I forgot/ignored separating him from the rest of the population. All of a sudden, a message pops up saying a citizen turned into a werepossum (which I learned doesn't pause the game like other !FUN! that happens in the game). They too were dealt with quickly, but that was only the beginning.
Soon, every other minute I was notified of citizens turning into werepossums. The hospital was a bloody mess. The military was even stationed in the hospital at one point. When nothing happened, they were dismissed from their orders, and soon the spiral began again. The manager had to order more and more coffins be made, until the fortress ran out of non-economic rock. How could the fortress run out of rock with all the mining? Turns out the miners were fucking around in the tavern instead of doing their jobs. Several designated areas were waiting to be mined out. Did I mention the one dwarf who had withdrawn from society and just walked around doing fuck all.
The chief medical dwarves were killed in these wereposssum incidents. Twice. Citizens became too horrified to put items into stockpiles. Miasma was starting to spread. The military squads were slaughtered down to three people. In the midst of all this destruction and chaos, visitors were petitioning to join the fort. Imagine you're a human bard or poet, you're in a tavern covered in blood and strewn with corpses, you hear dwarves throwing tantrums in the distance, and you say, "Yeah, I wanna join this party!" I accepted at least three petitions from performers. On the bright side, warriors were also petitioning to join, so I was able to fill my depleted elite squad of soldiers with mercenaries.
Finally, I tried to do some more active damage control. Part of this was retiring the tavern in an attempt to get the lazy miners to do their jobs. This was folled by the transformations, deaths, and tantrum spirals kicking into high gear. The 80+ population fell down to 7. The third and last elected mayor, Tirist, snapped. She threw a tantrum and ran around the fort, eventually arriving at the tavern. Remember those bards/poets who became residents earlier? At that moment, they turned into werepossums and killed her. A werehorse showed up and attacked the pen/pasture area, scattering all the animals. In the end, there was only a single dwarf left, Etur, a mason. He drank, ate, and sunk into depression. He threw a tantrum, and ran in and out of the tavern, where the merry performers were playing music like nothing happened. He calmed down at some point, wandered, entered miasma, and started tripping around outside aimlessly. I gave him the mining job, just to give him something to do, before I finally put the fort out of its misery and abandoned it.
According to legends, Etur fled into the jungle. A new entity formed in the ruins of Townpage.