In the early days of germ theory, contagious diseases were thought to be caused by fungi or bacteria. In the 1890s, Dmitri Ivanovski filtered extracts from diseased tobacco plants and discovered that the disease could be transmitted to new plants through the filtrate. He concluded that the disease was caused by particles smaller than bacteria: the tobacco mosaic virus.

The statement, "In the early days of germ theory, contagious diseases were thought to be caused by fungi or bacteria. In the 1890's, Dmitri Ivanovski filtered extracts from diseased tobacco plants and discovered that the disease could be transmitted to new plants through the filtrate. He concluded that the disease was caused by particles smaller than bacteria; the tobacco mosaic virus", is partially correct. Dmitri Ivanovski discovered that filtered extracts from diseased tobacco plants could be transmitted to new plants through the filtrate. However, it was Dutch microbiologist Martinus Beijerinck who, while replicating Ivanovsky's experiments, proved that the filtrate contained a new infectious agent. Beijerinck named this agent a virus.