The diseases they cause are grim. Mad Cow Disease is the most famous, but kuru also possesses a certain notoriety thanks to its unorthodox mode of transmission. Although uncommon, prion diseases are incurable and bring dementia swiftly followed by death. In the case of spontaneous Creutzfeldt-Jakob Disease (sCJD), the most common prion disease, half of patients are dead within six months of symptom onset. That figure reaches 95% within a year. In a particularly vexing twist, prions are also nearly impervious to destruction, even when attacked using a strenuous combination of disinfectants, heat, and pressure.
As you can imagine, this makes prions difficult to eliminate from infected tissues and equipment. In addition to corneal transplants, sCJD has also been transmitted by transplants of brain tissue called dura mater, growth hormone from cadavers, and perhaps most worryingly, neurosurgical instruments. So any method that might throw prions into the path of uninfected people deserves scrutiny.
.Prions, Nearly Indestructible and Universally Lethal, Seed the Eyes of Victims – Scientific American Blog Network