"This evidence has led to the now widely accepted prion theory, which states that the cellular protein PrP is the sole causative agent of prion diseases; there is no nucleic acid involved. The theory holds that PrP is normally in a stable shape (pN) that does not cause disease. The protein can be flipped, however, into an abnormal shape (pD) that does cause disease. pD is infectious because it can associate with pN and convert it to pD, in an exponential process--each pD can convert more pN to pD.
(above is from the following URL)
https://www.scientificamerican.com/article/what-is-a-prion-specifica/
