Discussion on Protista

Prions: The name of which is derived from the words "protein" and "infection" -- are abnormal formations of a naturally occurring protein, PrP, that is coded in the genes of all mammals. Prions transmit diseases called transmissible spongiform encephalopathies, or TSEs. These diseases cause small "holes" in brain neural tissue that can be seen when viewed under a microscope. This causes the tissue to take on a spongy texture and appearance. Scrapie, a disease of sheep, was the first TSE discovered; others, such as bovine spongiform encephalopathy, or BSE -- commonly known as "mad cow disease" -- have become serious health and economic concerns.
Viroids: Normal viruses contain a protein capsule in which either DNA or RNA is used to synthesize the elements of the virus for proliferation and propagation. Viroids, however, consist of only a single RNA strand. They do not code for proteins in the way that viruses do; rather, they rely for reproduction on proteins and enzymes already present in host cells. Viroids affect plants; some do not display symptoms in infected crops. Crops that may be affected by viroids include coconuts, tomatoes, potatoes, cucumbers and citrus fruits.