Viroids and prions are infectious agents that are not living entities but can still transmit disease. Prions are abnormal forms of the protein PrP; they transmit several diseases that affect neural tissues in animals and humans. Viroids differ from viruses in that they lack a protein coat; they are merely circular, single-stranded RNA fragments that replicate on their own inside plant cells.
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.
Prions in Humans
In humans, TSEs cause several fatal diseases that affect the brain and its function. Kuru was first discovered amongst a cannibalistic tribe in Papua New Guinea. Creutzfeldt-Jakob disease, the most common TSE in humans, "affects about one in every million people each year," according to the National Institute of Neurological Disorders and Stroke. Other TSEs include fatal familial insomnia, Gerstmann-Staussler-Scheinker disease and variant Creutzfeldt-Jakob disease. These diseases can be passed hereditarily or through contact with infected tissues or bodily fluids. Normal sterilization techniques do not destroy prions.
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.
Viroids in Humans
Viroids do not cause disease processes in humans or animals; their effects are limited to plants. However, a similar infectious agent, a virusoid, causes type D hepatitis, an exceptionally virulent form of that disease. Virusoids are single-stranded, circular RNA molecules, like viroids, but they require a helper virus to infect. In the case of hepatitis D, the virusoid requires the protein coat of hepatitis B for transmission.