Who discovered the tobacco mosaic virus?

Dmitri Ivanovsky discovered tobacco mosaic virus by showing that infected plant sap remained infectious after filtration.

In 1892, Ivanovsky investigated a disease that produced a mottled pattern on tobacco leaves. He passed sap from diseased plants through a fine Chamberland porcelain filter, which was expected to retain bacteria. The filtered liquid still transmitted the disease, providing early evidence for an infectious agent smaller than known bacteria.

Ivanovsky did not initially establish the modern concept of a virus. He continued to consider the possibility that the agent was an unusually small or filter-passing bacterium. In 1898, Dutch scientist Martinus Beijerinck independently repeated the work and argued that the agent represented a new, non-bacterial category; he used the term virus.

That distinction explains a frequent mix-up. Ivanovsky is generally credited with discovering the first evidence of a virus, while Beijerinck is credited with recognizing and defining its viral nature. Tobacco mosaic virus later became the first virus crystallized, in 1935.