Russian microbiologist who, from his study of mosaic disease in tobacco, first detailed many of the characteristics of the organisms that came to be known as viruses (virus). Although he is generally credited as the discoverer of viruses, they were also independently discovered and named by the Dutch botanist M.W. Beijerinck only a few years later.

While a student at St. Petersburg University, Ivanovsky was asked in 1887 to investigate “wildfire,” a disease that was infecting tobacco plantations of the Ukraine and Bessarabia. In 1890 he was commissioned to study a different disease that was destroying tobacco plants in the Crimea. He determined that the infection was mosaic disease, which was believed at the time to be caused by bacteria. Using a filtering method for the isolation of bacteria, Ivanovsky discovered that filtered sap from diseased plants could transfer the infection to healthy plants. Further researches led Ivanovsky to conclude that the causal agent was an exceedingly small parasitic microorganism that was invisible even under great magnification and that could permeate porcelain filters designed to trap ordinary bacteria. He differed from later researchers of viruses only in his supposition that the pathogenic agent in question was a minuscule bacterium, rather than an entirely new and different type of organism. He published his findings in an article, “On Two Diseases of Tobacco” (1892), and a dissertation, Mosaic Disease in Tobacco (1902).

Ivanovsky continued his formal education in botany and taught plant anatomy and physiology (1896–1901) but did no further work in virology. In 1908 he went to teach at Warsaw University, where he studied the process of photosynthesis, with particular attention to chloroplasts (chlorophyll-bearing structures) and to the role of pigments in plant leaves.