I've recently had a story similar to this, like your giant worm with deadly dust I had a similar unwanted guest that came to visit my fortress.
It was a demon in the shape a humanoid gecko which was the leader of a nearby goblin race who decided to invade with his goblin army, after slaying the demon and winning the battle my dwarven soldiers returned to the fort, I proceed to forget about the demon and his ill lead assault.
Soon after I received a message about a stray cat that had been found dead, I ignored the message since there was a after all a battle, and it was properly strangled to death by a goblin or something, plus its only a cat who cares right? However then I started receiving more sinister messages about chickens and other stray poultry appearing up dead, as well as more cats. I started looking into combat logs, no nothing there. I was starting to get perplexed, until my first dwarven victim appeared. After seeing the message "Urist Mc-somthing-or-other cancels job because of external hemorrhaging" I zoomed in on my makeshift hospital, and as it appears he was doing just that. I watched as blood flew on the bed,walls, hospital floor and close to nearby solider's on medical leave from the previous battle, I like to imagine that those drawves were screaming, trapped to their beds as there close friend and colleague a few feet away was interrupting a stream of poisonous blood that would infect and kill them on touch.
As he proceeded to die, I proceeded to look at dwarf's equipment section, and yes he had the blood of a stray cat touching him somewhere, indicating that the dead animals and this were related somehow. After looking around I went and found what appeared to be demon extract littered over the previous battlefield, the source located I proceeded to issue a quarantine, no one allowed outside the fort. I set up a dwarf wading pool to contain the disease plus round up every free roaming animal I had to either (a butcher them or (b throw them in a pit to make sure they wouldn't spread the disease.
Two more dwarves died from the disease, one being an armourer, while not being legendary he was acceptable in his craft. The Hospital beds littered in blood were dumped as well as any stray animals corpse laying around. After a few more animals dying as well as another dwarf the disease began to die out, and the quarantine was lifted. As it turned out the disease caused enough hemorrhaging to kill anything smaller then a dwarf, and was not 100 percent lethal towards dwarves as one of my dwarf's managed to survive it somehow, either with the help of surgeons or by shear luck.
While not as deadly as your super Ebola, this did give me a bit of a scare at the time, as it was only my second experience with a disease that spread throughout my fortress, that other being some symptom caused by some deadly dust that made miasma appear from and follow dwarves around but no other result then that. Anyway I hoped you enjoyed that story