CJD is thought to be caused by a protein called a prion. A prion causes normal proteins to fold abnormally. This affects the other proteins' ability to function.
There are several types of CJD. The disorder is very rare, occurring in about 1 out of 1 million people. It usually first appears between ages 20 and 70. Symptoms usually start in the late 50s.
CJD can be grouped into two types: classic or new variant disease.
The classic types of CJD are:
- Sporadic CJD makes up most cases. It occurs for no known reason. The average age at which it starts is 65.
- Familial CJD occurs when a person inherits the abnormal prion from a parent (inherited CJD is rare)
Classic CJD is not related to mad cow disease (bovine spongiform encephalitis).
However, variant CJD (vCJD) is a form of the disease that is related to mad cow disease. The infection that causes the disease in cows is believed to be the same one that causes vCJD in humans.
Variant CJD accounts for less than 1% of cases, and it tends to affect younger people. However, fewer than 200 people worldwide have had this disease.
Variant CJD can be caused by exposure to contaminated products. Other vCJD cases have occurred when people were given corneal transplants, other tissues, or blood transfusions from infected donors. It may also have been caused by contaminated electrodes used in brain surgery (before instruments started to be properly disinfected).
CJD may be related to several other diseases caused by prions, including:
- Chronic wasting disease (found in deer)
- Kuru (seen in New Guinea women who ate the brains of dead relatives as part of a funeral ritual)
- Scrapie (found in sheep)
- Other rare human diseases, such as Gerstmann-Straussler-Scheinker disease and fatal familial insomnia