Prions

Not all infectious diseases are transmitted by bacteria or viruses. Some neurological diseases, such as Creutzfeldt-Jaakob disease (CJD) or mad cow disease are in fact caused by agents called Prions, which are of similar size to viruses but are made up of only protein ^[1] These diseases are can be called Prion diseases or transmissible spongiform encephalpathies (TSE). ^[2] 

Prions have these characteristics:

1.  "The transmissible agent consists of aggregated forms of a specific protein" ^[3].
2. These protein aggregates cannot be degraded by the agents that degrade most other proteins ^[4].
3. "The protein is largely or completely derived from a cellular protein called PrP, that is normally present in the brain" ^[5].

So an aggregated form of a protein (that is already present in the brain) is the infectious agent in prion diseases ^[6].

The pathalogic mechanism of prions is often associated with structural change. Exogenous prions cause the endogenous host proteins to undergo a structural change, rendering them functionless or harmful.

References

1. ↑ J. M. Berg et al. (2007) p 53, Biochemistry, Sixth edition, New York, W.H. Freeman and Company
2. ↑ http://www.who.int/bloodproducts/tse/en/
3. ↑ J. M. Berg et al. (2007) p 53, Biochemistry, Sixth edition, New York, W.H. Freeman and Company
4. ↑ J. M. Berg et al. (2007) p 53, Biochemistry, Sixth edition, New York, W.H. Freeman and Company
5. ↑ J. M. Berg et al. (2007) p 53, Biochemistry, Sixth edition, New York, W.H. Freeman and Company
6. ↑ J. M. Berg et al. (2007) p 54, Biochemistry, Sixth edition, New York, W.H. Freeman and Company