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CJD is no longer regarded as one disease, but as a group of four rather different forms of illness of a type called transmissible spongiform encephalopathies (TSEs), which affect animals and humans.The term BSE stands for bovine spongiform encephalopathy,which is the form of this illness seen in cattle.

The four types of CJD vary in the age of onset, their symptoms and some of the changes that occur in the brain. However, there are broad similarities between them and they all involve the same basic disease process – the accumulation of abnormal prion protein in brain tissue – as outlined in the previous section.

Sporadic CJD (sCJD)
This is the commonest form (accounting for about 85% of cases) but it is still very rare. It affects about one person in a million in the UK each year, resulting in about 50-60 cases annually (compared to Alzheimer’s disease, which affects nearly 10,000 people per million in the UK). Its cause is unknown, it occurs worldwide, mainly in middleaged and older people.Very occasionally, younger people are affected.

Genetic or familial CJD (g or fCJD)
This is very rare, with only a few cases each year in the UK, and is due to an inherited genetic defect (a mutation) of the gene called PRNP, which is responsible for producing normal prion protein.The mutant gene seems to lead to an abnormal or disease-related form of the protein.The abnormal gene is called "dominant." This is because, even though we have two copies of our genes, one from each parent, we only need to have one abnormal or mutated gene to develop the illness – the normal copy does not prevent it.This is why people diagnosed with genetic or familial CJD usually know that cases have occurred before in their family.

Iatrogenic CJD (iCJD)
These are cases resulting from accidental transmission of other forms of CJD by medical or surgical treatment (see What causes CJD?). Only a few cases of this occur each year in the UK, and to date, only about 250 cases worldwide.The Chief Medical Officer set up the CJD Incidents Panel in 2000 to advise on the management of possible exposure to CJD through medical procedures.

Variant CJD (vCJD; formerly nvCJD)
This form of CJD was identified in 1996 and was originally termed ‘new variant’ CJD (nvCJD).This is the only form of CJD which is believed to have resulted from BSE in cattle, and no case has been identified with symptoms beginning before 1994. Most cases have occurred in the UK, with only five other countries affected by September 2002: 127 cases had been confirmed in the UK, six in France, one in the Republic of Ireland, one in Italy, one in Canada and one in the USA. In contrast to sporadic CJD, it predominantly affects young people, often in their twenties.