The diversity of the conformations may have led to different forms of the prion diseases.An unusual secondary structure named alpha sheet has been proposed as the toxic constituent of amyloid precursor proteins. This idea is not widely accepted at present, but a fair amount of evidence has accumulated, especially recently, in its favor. Definition
The name amyloid comes from the early mistaken identification by Rudolf Virchow of the substance as starch (amylumcode: lat promoted to code: la  in Latin, from Ancient Greek: ἄμυλον, romanized: amylon), based on crude iodine-staining techniques. For a period, the scientific community debated whether or not amyloid deposits are fatty deposits or carbohydrate deposits until it was finally found (in 1859) that they are, in fact, deposits of albumoid proteinaceous material. The classical, histopathological definition of amyloid is an extracellular, proteinaceous fibrillar deposit exhibiting β-sheet secondary structure and identified by apple-green birefringence when stained with congo red under polarized light. These deposits often recruit various sugars and other components such as serum amyloid P component, resulting in complex, and sometimes inhomogeneous structures. Recently this definition has come into question as some classic, amyloid species have been observed in distinctly intracellular locations. A more recent, biophysical definition is broader, including any polypeptide that polymerizes to form a cross-β structure, in vivo or in vitro, inside or outside cells. Microbiologists, biochemists, biophysicists, chemists and physicists have largely adopted this definition, leading to some conflict in the biological community over an issue of language.
where 
  
    
      
        
          δ
          
            i
            ,
            j
          
        
      
    
    {\displaystyle \delta _{i,j}}
   denotes the Kronecker delta. The physical interpretation of the various terms in the above master equation is straight forward: the terms on the first line describe the growth of fibrils via monomer addition with rate constant 
  
    
      
        
          k
          
            +
          
        
      
    
    {\displaystyle k_{+}}
   (elongation). The terms on the second line describe monomer dissociation, i.e. the inverse process of elongation.