Introduction: Prion diseases or transmissible spongiform encephalopathies (TSEs) are rare, fatal and
incurable neurodegenerative disorders of humans and animals (Prusiner, 1998).
In humans, prion diseases occur with unique aetiology as sporadic, genetic or infectious
disorders. Sporadic cases of prion diseases, which account for the majority of casualties (up
to 85% of all cases), are of unknown origin; the genetic forms are less frequent (up to 15%),
while the infectious cases are extremely rare with an incidence of less than 1% (Prusiner,
2001). Creutzfeldt-Jakob disease (CJD), Gerstmann-Sträussler-Scheinker (GSS) syndrome,
Fatal Familial Insomnia (FFI) are examples of human prion diseases. In animals the disease
is mostly infectious and the mode of transmission is horizontal. Prion diseases include
scrapie in sheep and goats, bovine spongiform encephalopathy (BSE) in cattle, and chronic
wasting disease of deer, elk, and moose (Williams, 2005).
The agents responsible for prion diseases are infectious proteins named prions. The term
‘prion’ was coined when Stanley B. Prusiner introduced the concept of proteinaceous
infectious particles (Prusiner, 1982). Since the introduction of this once heretical notion,
mounting evidence has strengthened its validity.
In the next sections of this chapter we present and discuss the peculiar complexity of the
molecular pathogenesis of prion diseases in humans and animals.
