In terms of the definition of life viruses are not alive. They are complex associations of two macromolecules—protein and nucleic acid—but they are neither self-regulating nor capable of metabolism. Perhaps most crucial is their inability to reproduce independently. They may be crystallized and kept in an inert state in test tubes for long periods of time. Their talent lies in their ability to seduce living cells into manufacturing new viral material following the injection of viral nucleic acid into those cells. The genetic message of the virus literally captures the protein-synthesizing machinery of the cell which then carries out the bidding of the viral information tape. The cell also produces the nucleic acids required for viral replication. At one time, viruses were felt to be links in the stepwise increasing complexity of macromolecules on their journey toward becoming full-fledged cells. The more likely explanation offered by many virologists at this time is that viruses are degenerate products of more complex forms, even of once-living cells.
Forms resembling living organisms exist that are even simpler than viruses. In 1971, a scientist at the U.S.Department of Agriculture showed that infections of potato plants are caused by very small bits of circular RNA lacking a protein coat. These tiny lengths of naked RNA, soon found to cause a variety of diseases in flowering plants, were called viroids by their discoverer, T. O. Diener. Although viroids cause disease, they do not destroy the cells they parasitize. The mechanisms whereby they enter the cell and take over part of its polynucleotide synthesizing machinery are not fully known.
Several neurological diseases, including scrapie in sheep, arise from an infestation of central nervous system cells by a self-replicating protein called a priori. Prions have been isolated from diseased tissues but their mode of action and reproductive strategy are unknown.
