Creutzfeldt-Jakob disease (CJD) is a rare and fatal brain disorder that belongs to a group of human and animal diseases known as prion diseases. It is caused by an abnormal infectious protein called a prion that accumulates at high levels in the brain and causes irreversible damage to nerve cells. There are different types of CJD, including classic CJD, variant CJD, and familial CJD.

Classic CJD is the most common type of CJD and is not related to "mad cow" disease or variant CJD. It is a rapidly progressive, invariably fatal neurodegenerative disorder that leads to dementia and death usually within one year of onset of illness. Symptoms of CJD include loss of intellect and memory, changes in personality, loss of balance and coordination, slurred speech, vision problems and blindness, abnormal jerking movements, and progressive loss of brain function and mobility.

Variant CJD is a rare form of CJD that is related to bovine spongiform encephalopathy (BSE), also known as "mad cow" disease. It is believed to be caused by consuming meat from a cow that had BSE. Since the link between variant CJD and BSE was discovered in 1996, strict controls have been put in place to prevent meat from infected cattle entering the food chain.

Familial CJD is a rare inherited form of the disease that is caused by a mutation in the PRNP gene. It tends to develop slightly earlier in life than sporadic CJD, usually around age 50.

There is no known cure for CJD, so treatment aims to relieve symptoms and make the person as comfortable as possible.