Long ago it was thought that certain degenerative diseases, apparently immune to medicine, could be cured by sufficient clean cold air.
This lead to an exodus of victims to brighter climes. The sick thronged, muttering and promenading, by clear seas and cold hills. But, some doctors doubted the clarity of even these infusions.
They settled on caves.
Caves breath after all, and the deeper and larger the cave then the longer and more powerful the breath. What could be safer, more regenerating than to bask in the cool airs in the womb of the earth itself?
Things did not go as planned. Within a few weeks of total darkness the body's immune system drops to zero. This, and other strange qualities of the darkness lead to unexpected reactions in the interred, violent reactions.
When the plague victims were finally forced back into the cave system, a wall was built to hold them there. That was many many years ago, but they still wander the underground. The same individuals. You can sometimes hear their endless muttering as they engage in pleading negotiations with the disease that now preserves and animates them.