Different types

> 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 (this form of CJD is rare).

> Acquired CJD includes variant CJD (vCJD), the form related to mad cow disease; and iatrogenic CJD, in which the disease is sometimes passed through a blood product transfusion, transplant, or contaminated surgical instruments.

> Variant CJD is caused by eating infected meat. The infection that causes the disease in cows is believed to be the same one that causes vCJD in humans. Varient CJD causes less than 1% of all CJD cases. It tends to affect younger people. Fewer than 200 people worldwide have had this disease. Almost all cases occurred in England and France.