How do prions cause CJD?

In Creutzfeldt-Jakob disease and other prion diseases, the prion protein folds into an abnormal shape where the flat sheet structure replaces the helix, which means the protein can't do its normal job.

Unlike any other known protein, abnormal prion proteins are infectious. This was a radical discovery because proteins don’t contain the genetic material that allows viruses and bacteria to reproduce.

These misfolded proteins induce other prions to misfold. Then these misfolded prions build up in the brain and cause the infected brain cells to die. When the infected cells die, prions are released into normal tissue and go on to infect more cells without any reaction from the immune system. Eventually, large clusters of cells die leading to the mental and behavioral symptoms of prion diseases.

Prion diseases are the only known diseases that can be sporadic, genetic or infectious.