Prions are proteins that cause a number of invariably fatal neuro-degenerative diseases, which can be classified into two groups: genetic or sporadic diseases (GSD) and transmissible spongiform encephalopathies (TSE). Both types of disease require the development of both normal prion (PrP) and abnormal prion (PrP(sc)) which differs from PrP in having a tertiary structure rich in beta-sheets. In fact, PrP(sc) is a totally dehydrated protein with an anhydrous environment, probably a thin carbon dioxide gas gap, that is why it appears highly resistant to proteases, to chemical disinfectants in water phase except in certain conditions to sodium hydroxide and sodium hypochlorite, to heat and to radiation. GSD and TSE diseases differ in incubation time, primary symptoms, and nature of CNS lesions. This paper argues that diseases of the GSD type as inherited or hereditary metabolic disorders and diseases of the TSE type could be regarded as chemical poisonings. TSE is caused by a deficiency in the chemo-defense system (CDS), which is unable to destroy or eliminate PrP(sc). As a result, the immune defense system (IDS) accommodates PrP(sc) as an inert particle if not a virus lure and routes it through to the nervous central system and the brain via the body's lymphoreticular system. In TSE PrP(sc) acts inside the cells as a toxic disruptor of post-translational phase of PrP biosynthesis. Unfortunately, CDS and IDS appear unable to neutralize PrP(sc).