A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. The cited article says viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea, that they are found in almost every ecosystem on Earth and are the most numerous type of biological entity, and that since the first discovery of a virus, the tobacco mosaic virus, in the 1890s, more than 16,000 of the millions of virus species have been described in detail. Pests and diseases recorded in the atlas as caused by a virus are grouped under this concept.