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Prions have not yet been found in parasites, but there is little doubt that they exist in these organisms also. I am including a Chapter on prions, since this discovery has made a major impact on modern biology (and also because I find it fascinating).
The study of prion diseases began with the sheep disease, scrapie.
In recent years, a new prion disease disease has appeared in wild elk and deer in the United States and Canada.
The first indication that a neurodegenerative disease in humans was caused by a protein came from the study of Kuru in New Guinea.
The study of Kuru and Scrapie led to the prion hypothesis of Griffith and Pruisner
A rare neurodegenerative disease in humans was shown to be able to be transmitted by contaminated hormone and transplants, again suggesting the presence of an infectious agent:
A neurodegenerative disease was recently seen in cattle in the U.K. This disease appeared to be spread by contaminated feed. There is some evidence that this disease can be transmitted by eating infected animal brain tissue. It appears however to have been contained by stopping the practice of feeding rendered sick animals to cattle.
These studies led to the discovery of the infectious agent: it was surprisingly an endogenous protein which could assume a special conformation.
The discovery that prions existed in yeast was important since it allowed experimental work to be performed.
A recent study that suggests a possible role of prions in memory