Viruses are much too small to be seen with any light microscope.  The largest smallpox virus is more than 5 times smaller than E. coli and 50 times smaller than a human red blood cell.

In 1892, Dimitri Ivanowsky demonstrated that the tobacco mosaic virius was a "filterable" agent of disease, since it could pass through a porcelain glass filter designed to trap very small bacterial cells.  The filtrate of infected sap from tobacco leaves could pass the disease to healthy uninfected plants.  Wendell Stanley isolated and characterized this virus in 1935.

The name virus, the Latin term for "poison," became standard for these pathogenic agents by the late Nineteeth Century, replacing the phrase contagium vividium fludium, also Latin for "contagious living fluid."