Amyloids are aggregates of proteins that become folded into a shape that allows many copies of that protein to stick together, forming fibrils. In the human body, amyloids have been linked to the development of various diseases. Pathogenic amyloids form when previously healthy proteins lose their normal physiological functions and form fibrous deposits in plaques around cells which can disrupt the healthy function of tissues and organs.
Such amyloids have been associated with (but not necessarily as the cause of) more than 50 human diseases, known as amyloidosis, and may play a role in some neurodegenerative disorders. Some amyloid proteins are infectious; these are called prions in which the infectious form can act as a template to convert other non-infectious proteins into infectious form. Amyloids may also have normal biological functions; for example, in the formation of fimbriae in some genera of bacteria, transmission of epigenetic traits in fungi, as well as pigment deposition and hormone release in humans.Amyloids have been known to arise from many different proteins and polypeptides. These polypeptide chains generally form β-sheet structures that aggregate into long fibers; however, identical polypeptides can fold into multiple distinct amyloid conformations. The diversity of the conformations may have led to different forms of the prion diseases.