No person discovered the viruses. Instead an understanding of the viruses evolved in the late 1800s with the general understanding of the germ theory of disease. Different diseases had recognizable paterns and, although a bacterium, Protozan, fungus, or other agent could be isolated for most diseases, some diseases had no identifiable agent. Many of these diseases would become the viral diseases.

A mechanical device, the filter, led to the early interest in viruses. In 1884, Charles Chamberland, an associate of Pasteur, devised a porcelain filter to trap the smallest known bacteria, and in 1892, the Russian pathlogist Dimitri A. Iwanowskiused Chamberlands filter in his studies on tobacco mosaic disease. In tobacco mosaic disease, the tobacco leaves shrivel and assume a mosaic(patchwork) appearance before dying. Iwanowski filtered the crushed leaves of a diseased p;ant and found that the clear sap dripping from the filter contained the infectious agent. Unable to see any microorganisms, Iwanowski reported that a "filterable virus" was the agent of disease. This merely meant that the unseen agent, whatever its nature, would pass through a bacterial filter. At the time it was a remarkable discovery, because scientists could scarcely comprehend anything smaller than bacteria as disease agants. Six years later, in 1898, Martinus Beijerinck of Delft(Van Leeuwenhoek's city) repeated Iwanowski's work and added considerably to the knowledge of viruses. Beijerinck found that the tobacco mosaic virus would pass through a gel and remain active in dried leaves and soil. By testing the activity of many dilutions, he was able to demonstrate the potency of this viral fluid and show that it was inactivated by boiling. Beijerinck concluded that the disease agent must be a "contagious living fluid (contagium vivum fluidum) rather than a solid object.

Before the excitement of these discoveries subsided, Pual Frosch and Friederich Loeffler reported in 1898 that foot and mouthdisease was cause by a filterable virus. This implied that an invisible agent could be transmitted among animals as well as plants. This invisible agent is now what we know as virus