Adapted from the book, Scientists and Discoveries:
In 1892, Dmitry Ovanovsky, a Russian botanist, investigated the sap from tobacco plants affected by mosaic disease. This is a condition that stunts the growth of tobacco plants and mottles their leaves in a characteristic mosaic pattern (hence the name). He passed the sap through filters so fine that they would trap all known bacteria. The sap retained its ability to produce the disease in new plants. Ovanovsky thought there was some flaw in his experiment, not realizing that viruses (which had not yet been discovered) were at work.