Prions are infectious agents composed entirely of a protein material that can fold in multiple, structurally distinct ways, at least one of which is transmissible to other prion proteins, leading to disease in a manner that is epidemiologically comparable to the spread of viral infection. Prions composed of the prion protein (PrP) are believed to be the cause of transmissible spongiform encephalopathies (TSEs) among other diseases."

Source : Wikipedia

Prions don't contain DNA, so aren't normally considered to be in any sense 'alive' (see: Defining Lifeplugin-autotooltip__plain plugin-autotooltip_bigDefining Life

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There isn’t, as yet, any definition of a 'lifeform' which can withstand logical scrutiny.

Exceptions can easily be found for any definition so far presented. For example (from The American Heritage Dictionary of the English Language). Nevertheless, prions can readily replicate within organisms. The mechanism(s) by which they do so has not yet been determined.

There are currently two main theories :

• The 'heterodimer' model
• The 'fibril growth and fibril breakage' model

Neither explains all the observed manifestations. See: Prion replication mechanism, Wikipedia.

Also see : TSEs (Transmissible Spongiform Encephalopathies)plugin-autotooltip__plain plugin-autotooltip_bigTSEs (Transmissible Spongiform Encephalopathies)

Transmissible Spongiform Encephalopathies (TSEs) are severe degenerative brain diseases which affect a wide range of mammals - they include bovine spongiform encephalopathy (BSE or "Mad Cow Disease"),