Where do prions come from?

PrP-sen is produced by normal healthy cells, and is present mainly in neurons in the brain.
The exact function of PrP-sen is not known, but there is evidence that it may be involved in communication between neurons, cell death, and controlling sleep patterns.
The second type of prion protein, known as PrP-res, is the disease-causing form, and organisms with it develop spongiform diseases.