Which virus was the first to be discovered and characterized?

The first virus to be discovered and characterized was the Tobacco mosaic virus (TMV). In 1892, a Russian scientist named Dmitri Ivanovsky observed that extracts from infected tobacco leaves remained infectious even after passing them through a filter that retained bacteria. Martinus Beijerinck further studied this filterable agent and named it a "virus". In 1935, Wendell Stanley crystallized the virus and determined its structure, which was a major breakthrough in the field of virology. TMV is a plant virus that infects tobacco and other plants, causing a distinctive mottling or discoloration of the leaves.