Prion diseases or transmissible spongiform encephalopathies are rare neurodegenerative diseases. They occur when a normal prion protein, found on the surface of many cells, take on an abnormal structure. This abnormally structured prion is then able to induce the conversion of normally structured prions into abnormally structured ones. The latter accumulate in the brain and cause extensive brain damage. Several human and animal prion diseases are known, with CJD being one of them.

Abnormally structured prions are very stable and cannot be destroyed by standard chemical and physical procedures.

There are 4 known forms of CJD:

1. Sporadic  CJD happens without a known cause. It is the most common form of CJD, accounting for almost 90% of cases.
2. Familial  CJD is caused by an inherited mutation in the gene coding for the prion protein. It is the second most common form of the disease, accounting for 8% of cases.
3. Iatrogenic  CJD is caused by the transmission of the abnormally structured prion from an organism to the affected person during a medical procedure. It has mainly been documented in recipients of contaminated human growth hormone or dura mater. Due to increased awareness, iatrogenic CJD occurs very rarely nowadays and the cases that still occur have been infected a long time ago.
4. Variant CJD has first been found in 1996 in the UK. It is linked to the consumption of meat of cattle infected with bovine spongiform encephalopathy (or mad cow disease). Variant CJD cases are generally younger than typical CJD cases. Worldwide, 230 cases have been detected, the majority of them living in the UK. The epidemic reached a peak in 2000 and in recent years cases occur only very sporadically. This form has never been detected in Belgium.

CJD results in rapid brain degeneration. This degeneration results in a range of neurological (e.g. problems walking, speaking, seeing) and psychological symptoms (e.g. confusion, memory loss, depression). Disease progression is fast, usually less than a year, although variant CJD is known to have a slightly longer duration, up to 2 years. The disease inevitably results in the death of the patient.

CJD is not contagious in the strict sense. However, it can be transmitted through the consumption (see variant CJD) or injection of infected brain material (e.g. during medical procedures; see iatrogenic CJD).

Additionally variant CJD has been shown, on four occasions, to be transmissible through blood transfusion. There exists no evidence that non-variant CJD is transmissible through blood.

Although several test results may be indicative of CJD, the only way to confirm the diagnosis of CJD, and to distinguish between variant CJD and non-variant CJD is by brain autopsy, or, more rarely, brain biopsy.