Viroids are smaller than viruses, self–replicating particles that were discovered by T.O. Diener (1971). Viroids are infectious RNA particles that are devoid of protein coat. The molecular weight of RNA in viroid is low in comparison to viruses. Potato spindle tuber is a viroid causing disease.

DISCOVERIES OF VIROLOGY: 

• Term virus (means venom or poisonous fluid) was coined by Pasteur (1880).
• D.J. Ivanowsky (1892) recognized certain microbes as 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 this fluid as Contagium vivum fluidum (infectious living fluid).
• W.M. Stanley (1935) crystallized TMV (Tobacco mosaic virus) for the first time. He showed that viruses could be crystallized and crystals consist largely of proteins.