ABSTRACT
Viruses are among the simplest and smallest of currently known living organisms. In fact, because of their simplicity, there is some debate as to whether viruses should be considered living. Most viruses consist of both protein and nucleic acid. Viroids (plant pathogens that consist solely of RNA) and prions (agents that appear to consist solely of protein) are exceptions. Viruses generally exist in two forms. The actively replicating
virus inside an infected cell is the form that may be considered ‘‘alive.’’ The extracellular form of the virus is known as the virion. The virion is analogous to a seed or spore. It generally is a stable ‘‘crystalline’’ structure whose primary function is to protect the genetic material until the nucleic acid reaches the interior of a suitable host cell. No virus is capable of growing by itself. All must make use of macromolecular ‘‘building blocks’’ (amino acids, nucleotides, and, in some cases, lipids) and employ enzymes found within living cells. Thus, all viruses are obligate intracellular parasites.
