A nasal brush test can rapidly and accurately diagnose Creutzfeldt-Jakob disease (CJD), an incurable and ultimately fatal neurodegenerative disorder, according to a study. CJD is a prion disease. These diseases originate when, for reasons not fully understood, normally harmless prion protein molecules become abnormal and gather in clusters. Prion diseases affect animals and people. Human prion diseases include variant, familial and sporadic CJD. The most common form, sporadic CJD, affects an estimated 1 in one million people annually worldwide.