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Dmitri Ivanowski publishes the first evidence of the filterability of a pathogenic agent, the virus of tobacco mosaic disease, launching the field of virology. He passes the agent through candle filters that retain bacteria, but he isn't sure that the agent is a unique organism.
When Did Virology Start, ASM News 62, 1996. p.142 [pdf]
Tobacco Mosaic Virus and Its Contributions to Virology, ASM News 65, 1999.p.675