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Martinus Beijerinck recognizes "soluble" living microbes, a term he applies to the discovery of tobacco mosaic virus. He demonstrates that juice pressed from tobacco leaves that had been filtered free of bacteria retains the ability to cause disease in plants even after repeated dilutions. He calls the disease agent "contagium vivum fluidium" or contagious living fluid.
Martinus Beijerinck (1851-1931), ASM News 62, 1996. p.539 [pdf]
Beijerinck, M. 1899. Ueber ein Contagium vivum fluidum als Ursache der Fleckenkrankheit der Tabaksblatter. Centralblatt fur bacteriologie und Parasirenkunde, Part II, 5: 27-33. In Milestones in Microbiology: 1556 to 1940, translated and edited by Thomas D. Brock, ASM Press. 1998, p153 [pdf]