The name virus, that means venom or poisonous fluid, was given by Dmitri Ivanowsky (1892). He recognized certain microbes as the causal organisms of the mosaic disease of tobacco. These were found to be smaller than bacteria because they passed through bacteria-proof filters. M.W. Beijerinek (1898) demonstrated that the extract of the infected plants of tobacco could cause infection in healthy plants and called the fluid as contagium vivum fluidum (infectious living fluid). W.M. Stanley (1935) showed that viruses could be crystallized and crystals consist largely of proteins.