In case you were wondering, here's another reason not to mess with the zombies: their touch pretty much turn anyone they touch into the walking dead. As soon as someone touches them with their bare hands, or takes a hit from them, it's all over for the victim, because they instantly recieve the syndromes of both the fetid muck and nefarious mist, resulting in instantaneous swelling of every aprt of the body, and subsequent necrosis.
I just played through to the fort's end; 8-9 migrants made it inside, then one mysteriously decided to go back outside and died. Goodbye experienced weaponsmith. Months later, the elves show up and get swarmed immediately, dying horribly. Then the dwarven caravan shows up and the hammerdwarf guards kick ass, leaving nothing behind. Except I accidentally let a zombie dwarf and zombie dingo inside, who promptly killed a dwarf outright and sent three others to the hospital. Somehow, none of them died of the rot, but strategia's baby got bit by a dingo and did exactly that. So, logically, a fucking kobold or something picks the locks on the doors down south Spish just warned us about, and zombies start piling in. First it was just one zombie dwarf, who singlehandedly killed four more dwarves. Then an ogress corpse meanders in to reinforce him. The caravan packs up at this point and as they're leaving they deal with the zombie problem, but it doesn't matter because 3/4 of the 4 dwarves are resting, and Strategia, the only one left in good health, is tantruming herself into a hole. She and one of the injured go melancholy, and the 3 injured all die of thirst while Stategia gets pwned by yet another zombie.
Yeah, just don't open the doors except to get those migrants inside.
On another note, make the sludge-filled tundra a "restricted access" traffic zone, just to help dwarves understadn not to go there, ever.