The effect of Creutzfeld-Jacob disease (CJD) is so devastating that it eventually makes the brain look like a sponge: fine networks of fibres replace many of the cells. For this reason, CJD and related illnesses are also known as 'spongy brain diseases'. Many scientists believe that these diseases are caused by prions: agents made entirely out of protein (unlike bacteria and viruses, which also have genes). Infectious prions that cause CJD are variants of normal brain prion proteins. There is no cure for CJD.