Mondo Description Sporadic Creutzfeldt-Jakob disease (sCJD) is the most common type of CJD, accounting for around 85% of cases. The precise cause of sporadic CJD is unclear, but it's been suggested that a normal brain protein changes abnormally ('misfolds') and turns into a prion. Most cases of sporadic CJD occur in adults aged between 45 and 75. On average, symptoms develop at age 60-65 years.