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As described above, the overwhelming majority (85%) of CJD cases are sporadic. Their cause is unknown, and prevention impossible.

The number of cases of iatrogenic CJD is small and the risk from this source is being reduced by identifying medical procedures that are most likely to cause problems and taking steps to avoid this happening. For example, growth hormone and dura mater prepared from humans are no longer used in medical treatment.

As discussed above (Can I get CJD from eating meat?) circumstantial evidence that variant CJD passed from cattle to humans led to changes in methods of rearing livestock and meat preparation.This included:

• In 1998, a ban on the use of meat and bone meal in animal feed.
• In 1989, a ban on inclusion of potentially infected material from the human food supply.
• In 1995, a ban on inclusion of vertebral column from mechanically recovered meat products.
• In October 2000, a ban on the use of a wide range of bovine (cow) body parts in the food chain including the entire head (except for the tongue), the brain, eyes and tonsils; the spleen; the intestines and the spinal cord of animals aged over six months and the vertebral column, (which houses the spinal cord and related structures) of animals aged over 30 months.

The effects of such changes on rates of disease will not become clear for some time because the length of incubation between exposure to an infectious agent and the start of the disease is not known. Estimates suggest that it may range from a minimum of five years to 30 years or even longer.

One of the key differences between variant CJD and the other forms is that organs such as the tonsils, spleen and appendix contain abnormal prion protein, rather than just the brain and spinal cord. This is reflected in the precautions taken in meat production, as explained above, and operations on such organs could also theoretically pose a risk of iatrogenic transmission if surgical instruments are re-used. However, as mentioned above, recognition of this possibility has led to steps being taken to reduce the risks.

It is important to remember that there is no evidence that CJD is infectious by airborne droplets (i.e. during colds or flu), contact with skin or blood or sexual intercourse.