In an affected animal, the brain develops tiny holes, making the animal lose control over its movements and changing its behavior. Mad cow disease is in a new class of infectious agents called prions. The disease is caused when a normal prion protein folds into an abnormal shape and no longer breaks down inside the body. Such misfolded proteins produce havoc in nervous tissue, often leaving sponge-like holes in many parts of the brain. When an animal or person eats or is exposed to tissues containing abnormal prions, their own prion proteins are converted into the deadly abnormal form.
Prions can also misfold on their own accord and give rise to disease.
Occasionally, genetic mutations lead to prion diseases that run in families. Mad cow disease has jumped species - from sheep to cows to humans. It's believed that mad cow disease is spread to people when they eat infected beef products. Since the first human cases were diagnosed in 1996, more than 80 people have died of vCJD in Britain and three in France. New variant CJD begins with psychiatric symptoms such as anxiety and depression.