From Lost Fables Wiki
Extended time in the Primordial Dream tends to eventually cause these cysts to form.
Spending too much time in the Primordial Dream can sometimes cause it, eating one of the pearly cysts from the body of a host will also spread the infection.
Only non-shaman beastfolk may carry this disease.
Mindcysts are a form of neurological fungal infection which can actually be fairly beneficial to those who host it, though it also comes with some serious downsides. On the upside, a person host to Dreamcysts can always choose to enter the Primordial Dream while they sleep, always remember their dreams (and nightmares) in perfect clarity, and, after about a month of hosting them, can always dream lucidly when they choose to not enter the Primordial Dream. The cysts allow you to contemplate and understand Truths with far more ease, and they make it far more unlikely for their hosts to accidentally kill themselves upon the first time that they learn and use a Truth. The downsides are that they almost immediately give their hosts a nasty headache, which more or less persists for every waking moment until the day they die, and the host will slowly (after about a year) begin to experience neurological degeneration, resulting in the shakes, which sometimes escalate into seizures, and in later life, a slow downward spiral into dementia. Many who confirm that they have them simply prefer to use themselves up casting magic rather than suffer such a fate. If an autopsy is performed on someone who dies with Dreamcysts, the coroner will find the brain to be filled with pearly white cysts clinging to their brain matter, likely the cause of the headaches, seizures, and neural degeneration.
Though the pain of Dreamcysts and the degeneration are considerable, they always take more than 30 years to kill their host, and frequently more than 50.
Only through using Truths, or a magical cure-all can these be removed. Generally speaking, it also requires the use of a Truth to accurately detect the cysts.