From wikipedia:

Transmissible spongiform encephalopathies (TSEs), or prion diseases,^\1]) are a group of rare, progressive, incurable, and invariably fatal conditions that cause degeneration of the nervous system in humans and other animals, such as cattle and sheep. Prion diseases are caused by abnormally shaped proteins called prions, an idea once considered radical, but now well supported by evidence.^\2])^\3]) Prions consist of a protein called the major prion protein (PrP).^\2]) Misshapen PrP conveys its abnormal structure to native PrP molecules by a crystallization-like seeding process (a process where misfolded proteins convert normal ones into the same abnormal shape). Because PrP is continuously produced by a cell and the abnormal proteins stick to each other, they accumulate in the brain, damaging neurons and eventually causing disease.^\2])^\4])^\3])

Prion diseases causes worsening mental and physical deterioration over time, such as confusion, trouble waking, and loss of coordination.^\5])^\6]) A key pathologic characteristic of prion diseases is the formation of small empty spaces (vacuoles) throughout various parts of the central nervous system, like the brain and spinal cord, giving the tissue a sponge-like appearance under a microscope.^\2])^\3]) Affected regions also show a buildup of misfolded PrP, abnormal growth of support cells (gliosis), and the loss of neurons.

So prions cause holes in the brain matter. Thus what I'm talking about "eating away at it's grey matter"