Creutzfeldt-Jakob disease (CJD) is a rare, fatal brain disorder caused by prions. Prions are normal proteins that have changed their shape. Healthy proteins have a healthy shape, which allows them to function normally. The misshapen prion protein is unhealthy. Worse, the prion causes disease by making other proteins nearby change their healthy shape to the prion's unhealthy shape. In this way a prion makes more prions: it slowly turns healthy proteins into prions, too.
When symptoms eventually appear, CJD causes rapidly progressive dementia (mental decline) and involuntary jerking muscle movements called myoclonus. About 90% of people with CJD die within 1 year of diagnosis.
About 85-95% of cases of CJD are from sporadic mutations. The DNA in a brain cell is changed by the mutation, and makes an unhealthy prion protein. Less often, the mutation is hereditary (passed down from parent to child). Jewish people born in Czechoslovakia, Chile and Libya have a higher-than-average number of inherited cases of CJD. Inherited CJD does not cause symptoms until adulthood.
Prions also can be passed from an animal to a person, or from one person to another. Fortunately, it is hard to catch prion diseases. A type of CJD called "variant CJD" shows somewhat slower progression of brain injury and more psychiatric symptoms, and it tends to affect younger people. This type of CJD has been linked to eating beef from cattle with bovine spongiform encephalopathy (BSE), also called "mad cow disease." BSE is caused by prions.
In the 1990's, a small outbreak of variant CJD was described in the United Kingdom. It was caused by people eating beef infected with prions. Since then, changes in beef farming and processing practices have helped to limit the number of new cases.
It is extremely uncommon for CJD to spread from one person to another. However, in very rare cases CJD has been transmitted by a blood transfusion, by a medical procedure (because of contaminated equipment), by contaminated tissue (such as corneas that are used for transplant) or by injections of hormones extracted from human tissues.