So this has happened to me twice now and both times its ended my game.
It seems the Hearthlings just start loosing health randomly and then they get to their beds and slowly die. Did I miss the update where epidemics were added? 
Anyone know what could be happening to make them loose health and how I can fix it?
Are you by chance using Stonehearth CafĆ© mod? I had this issue and it was freaking me out, once I disabled Stonehearth CafĆ© and the Brewery mod that seemed to fix it. Donāt know if it has to do with changes that were made to the game that has somehow turned the thirst mechanic into a replica of the starvation health loss.
Darn that probably its then as I do have it installed
What is food poisoning now a built in feature of the Cafe mod?
Actually, that would be a neat mechanic to introduce in-game. It gives the Herbalist far more to do.
I dunno, lol. I just signed in and my Hearthlings started losing health like crazy and my clerics were spam healing their little hearts out. They just couldnāt keep up and follks were losinā health all over the place. I had received an error about thirst but it was an older save so I assumed it was from before the fix until things started getting scary. So I disabled the mod and that seemed to fix the problem for me.
And yeah it would be nice if after they got all the building mechanics ironed out, the new faction in, and all the classes they wanna add, if they included some ādisastersā when they work on weather like systems. Maybe one of your new hearthlings brings in a plague the herbalist needs to make meds for before it gets out of hand. Tornados that vacuum up any mats laying in open stockpiles and spreading them around the area. Fires that damage structures (would need to give the hearthlings the ability to just auto repair em).
I had that problem too
after I placed some wells and harvested water it stopped they were just thirsty I guess
Wells solved the issue. Also explains why they started dying once I got a cook, cause thats when I stopped harvesting berries 
Well dang, I had 4 wells and was mad harvesting to the point that all my barrels were full and I had water on the ground. My hearthlings were still losing health like crazy. Guess my save glitched and thatās why it threw an error message.