Creutzfeldt–Jakob disease
CJD is caused by an infectious agent called a prion.  Prions are proteins that are folded wrong.  Prions make copies of themselves by changing correctly folded proteins into misfolded shapes.  CJD causes brain tissue to become unhealthy very quickly.  As the disease destroys the brain, the brain develops holes.  The brain's texture changes and becomes like a kitchen sponge.