Cerebral Parasites are a highly infectious disease of psychic, micro-organisms. It is highly likely that they don’t have any physical form, but only exist in the mind. They are said to be born out of the destruction of Kalidnay.
They feed of psionic energies, particular off individuals highly-practiced in The Way. Although everyone is susceptible to the parasites.
Eventually, Cerebral Parasites eat the mind of the host and kill them. Symptoms show in adepts far earlier than less psionically-geared people, meaning it’s often too late once the disease has been identified.
Epidemics have been known to appear in the past, but due to the isolationist nature of the Tyr Region, it rarely spreads past cities.