Answer:
a virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism.[1] Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea.[2][3] Since Dmitri Ivanovsky's 1892 article describing a non-bacterial pathogen infecting tobacco plants and the discovery of the tobacco mosaic virus by Martinus Beijerinck in 1898,[4] more than 9,000 virus species have been described in detail[5] of the millions of types of viruses in the environment.[6] Viruses are found in almost every ecosystem on Earth and are the most numerous type of biological entity.[7][8] The study of viruses is known as virology, a subspeciality of microbiology.