Something invisible is killing my dwarves

[Cateinum]

All of a sudden some of my dwarves (and cats!) started bleeding. They die in a couple of minutes leaving me no clue what's going on. They don't fight, there are no hostile creatures on the map, no one reported any crimes, the biome is not evil. The only notification I get is that somebody has found them dead. It seems to only happen on the ground level, but I'm not sure, and with random dwarves all the time if I reaload the save. I managed to survive there for 7 years without such issues. Now I'm confused.

[AlP]

Do you get a message that your dwarves were found drained of all blood? In that case you have a vampire.

Otherwise, you might have some kind of forgotten beast syndrome or evil rain.

[Santa]

sounds like you've let dracula in.

[MintiM]

Check their items, maybe they're covered in forgotten beast blood or something, it might be toxic

[Cateinum]

Some of them had an "obsidian coating" idk if it's toxic or not, only one was covered in forgotten beast blood. Also I have a puddle of forgotten beast blood at the entrance, but it was killed ages ago.

[AlP]

Do you have any helmet snakes or bushmasters on your map? Maybe the dwarves got bitten.

Otherwise, it's probably because of the forgotten beast blood.

[The Trashman]

Originally posted by Cateinum:

Some of them had an "obsidian coating" idk if it's toxic or not, only one was covered in forgotten beast blood. Also I have a puddle of forgotten beast blood at the entrance, but it was killed ages ago.

Some FBs can have poisonous blood. If the blood is toxic, then it could be absorbed through bare skin. You could try pasturing an animal on the blood, like a dog. If it dies, then the blood is probably toxic

[Helios]

Originally posted by Cateinum:

All of a sudden some of my dwarves (and cats!) started bleeding. They die in a couple of minutes leaving me no clue what's going on. They don't fight, there are no hostile creatures on the map, no one reported any crimes, the biome is not evil. The only notification I get is that somebody has found them dead. It seems to only happen on the ground level, but I'm not sure, and with random dwarves all the time if I reaload the save. I managed to survive there for 7 years without such issues. Now I'm confused.

there is a option in setting so spilled blood is not transported something with splatter ect for some reasons i have troll blood lasting in my pen for years now buit maybe its from my jabberer.

[Erei]

Looks like the work of a contaminant. Most likely from a FB.
Sometimes those supernatural creature have an "effect". Like deadly dust. That effect will go on affected dwarves and do something. That's random and range from harmless (bad rash) to literal death (bleeding to death from every part of their body), and include the annoying "rot everything".
When a dwarf is affected it will spread it around by touching things mostly. Animals will help the propagation by walking in it.

To prevent that issue I always make "dwarven footbath". If a dwarf walk in water, it will clean all contaminant. And when they are in water they are harmless unless ingested. So, I make 3*3 footbath at every main fort entrance (and one for the hospital). You can't get inside without walking in one. And one in every cavern access for FB. You need exactly 3/7water (below and it will evaporate needing constant maintenance, above and they "swim" which mean they won't cross it if they can't swim).
To make a footbath, simply dig an area in the corridor, leave the ramp so they can walk in and out. Then the easiest way is probably to make the place a "pit" for water until you have enough water.

Having water and soap will help, if a dwarf is full of bad stuff they will "take a bath", which is just some water and soap. But for nasty FB stuff, it's not enough.

[The Hat]

Make sure all dwarves get shoes.

Makes sure the water source in the fort is not contaminated and people have time to wash.

Some people use showers or bathtubs https://dwarffortresswiki.org/index.php/DF2014:Cleaning

Make sure you have a couple of dedicated cleaners. In a big fort, separate out the cleaning labor into its own separate labor skill and dedicate 1-2 cleaners/janitors.

Some people use swimming baths or as others have said footbaths, often putting a swimming pool near a meeting hall can train dwarves in swimming for example.

You can really make cleaning elaborate and tbh its a greater enemy for me than sieges or raids, as you are finding out. New players often flood their forts too, and gremlins sometimes do it for them (if no safeguards are in place on water flow).

Consider this a whole game within a game :D, and also consider any FPS hit of anything too elaborate.

[Cateinum]

I cleaned up that bloody puddle and that seems to solve the problem. Thanks for the advice, I'll try to play with pools and dedicated cleaneres