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What is Virus: A Historical Review of Discovery of the Tobacco Mosaic Virus

: In the years before the advent of electron microscopy, where the “germ”, as in the dominating germ theory of disease, was confined to bacteria, how did virus, which remained invisible, come to be known? How did people manage to identify something as “virus”? This article approached these issues by studying the in-tellectual history of science in the discovery of the tobacco mosaic virus (TMV), while remaining fully aware that scientific research has its limitations and that knowledge resulting from it could be wrong. Specifically, this article reviews eight milestone discoveries that gradually revealed the TMV: 1) tobacco mosaic disease was transmissible by inoculating healthy plants with sap from the leaves of diseased plants (Adolf Mayer); 2) the agent that causes the disease was filterable (Dmitri Ivanowsky); 3) the agent was contagium vivum fluidum (Martinus Beijerinck); 4) foot-and-mouth disease in cattle was caused by the “minutest organisms” (Friedrich Loeffler & Paul Frosch); 5) the infective entity of the tobacco mosaic disease was not contagium vivum fluidum (Ivanowsky); 6) the disease was caused by some particle that could multiply itself in cells (Benjamin M. Duggar); 7) crystallization showed TMV as a protein (Wendell Meredith Stanley); and 8) TMV turned out to be a rod-shaped aggregate of protein containing RNA.