Ultrastructure research: the production of the defective phage PBSX in Bacillus subtilis 168 following induction with mitomycin C
There exists among genetic forms the simplest self-replicating biological entity; namely, the "viruses." To define the term "virus" is not easy, for viruses cannot be defined by common-sense criteria the way animals and plants can. The term "virus" was first used by the ancient Roman physicians to denote a poison of animal origin. The diseases caused by such poisons were called "virulent." During the late nineteenth century Ivanowsky found that these invisible infectious agents were much smaller than bacteria. Because they retained their infectivity after passage through a filter able to retard the passage of all then-known types of bacteria, these submicroscopic agents were termed "filterable viruses." The early twentieth century brought with it the discovery that viruses were subcellular entities capable of entering living cells and of reproducing only in such cells. The viruses were thus referred to as "obligate intracellular parasites" whose specific action was at the level of the gene.