The Bacculus disease is an illness that causes the person to slowly turn to stone and die from it. An example of this is Lymle Lemuri Phi's Grandfather, Ghimdo. His legs starting from his feet slowly turned to stone eventually killing him.
The disease came from the Epiphanies of Guidance, specifically one on Lemuris. It gave Ghimdo and many others in the villages on Lemuris this illness. The party attempted to cure the village from this but failed as instead of getting the source of illness in a symbology stone, they destroyed it. This killed most of the village including her Grandfather. Other than this, Bacculus is not mentioned until a similar disease is found on Roak; the "Stone Illness".