CJD is caused by an infectious protein - a prion. Proteins are molecules made up of acids, that help the cells in our body to function.

The exact role of proteins in our brain is unknown, but it is thought they may have something to do with our long-term memory.

Proteins begin as a string of acids, and then fold themselves into a three dimensional shape. This 'protein folding' allows them to perform useful functions within our cells.

Sometimes mistakes happen during the protein folding, and the protein cannot be used by the body. These misfolded proteins are normally recycled by the body, but sometimes they can build up and cause problems, like Alzheimer's disease.

Prions cause even more problems. Not only do they fold into the wrong shape, they enter other brain cells and cause normal proteins to misfold as well. This causes the brain cell to die, releasing more prions to infect other brain cells. Eventually, clusters of brain cells are killed and replaced with deposits of prions - plaques. These plaques produce small holes in the brain, causing it to become sponge-like. It is the damage to the brain that causes the mental and physical impairment, and eventual death, associated with CJD.

Prions can survive in neural tissue - such as the brain or the spinal cord - for a very long time, even after the person or animal has died.

In the case of variant CJD it is believed that the infectious prion was spread through cattle, as they were fed meat and bone mix (MBM) that contained traces of infected brains or spinal cords. Then the prion was spread from cattle to humans, through mechanically recovered meat, which also contained small traces of infected brains or spinal cords.

Both the use of MBM and mechanically recovered meat has since been outlawed.