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Creutzfeldt-Jakob disease (CJD), named after two German doctors, is a rare neurological illness.There are four different types of CJD, each with different causes. Most people had not heard of CJD before 1996, when it became the subject of media attention following identification of the fourth form, which was then called "new variant CJD" (nvCJD) and is now called variant CJD (vCJD).

Affecting mainly young people, often in their twenties, vCJD is the only form of CJD which is believed to result from people eating meat from cattle infected with BSE (bovine spongiform encephalopathy/ "mad cow disease").There is no absolute proof of the involvement of diet, but it is thought highly probable that products from affected animals cause infection with an abnormal form of a protein called a prion (short for "proteinaceous infectious particle").

The illnesses are potentially transmissible from affected individuals, but often only by very particular means, such as the consumption of, or injection with, brain tissue. It is thought that they are not contagious or transmissible by ordinary day-to-day contact with those affected, by nursing them, through airborne droplets or by blood or sexual contact.

The accumulation of abnormal prion protein (PrP) is a central feature of all four forms of CJD, which are sometimes referred to as "prion diseases." The same type of abnormal prion protein is seen in the brains of animals with BSE and those of people who have vCJD.

The normal form of prion protein (sometimes written as PrPC) exists in the body and brain without causing harm. Its role is not clear, and experiments on animals designed to find out its role have not been conclusive. However, some animals, which have been genetically modified so that they do not have the normal prion protein, do develop neurological problems.

The abnormal form of PrP (PrPSc) has various unusual properties, including resistance to being broken down by enzymes that normally control the body’s protein levels, and its tendency to form into clumps in the brain.These clumps or plaques (similar to those seen in other conditions such as Alzheimer’s disease) are a quite common feature of CJD, seen when the brain is examined after death.

As explained below, abnormal prion protein may be formed by a spontaneous change in the normal form or an abnormality in the gene that instructs the body to make it. In the case of vCJD, it is thought that abnormal prion protein got into some processed foods containing contaminated brain or spinal cord material and that this in turn started a cascade by which more and more prion protein was converted to the abnormal form, leading eventually to brain damage. (see also, Can I get CJD from eating meat?).