Viruses

A virus is an infectious sub microscopic creature that only reproduces inside of live cells. All living things, including plants, animals, and microbes like bacteria and archaea, are susceptible to virus infection. Since Dmitri Ivanovsky's 1892 publication revealing a non-bacterial pathogen infecting tobacco plants and Martinus Beijerinck's 1898 discovery of the tobacco mosaic virus, more than 9,000 viral species—out of the millions of different types of viruses in the environment—have been documented in detail. Viruses are the most common sort of living organism and may be found in practically all ecosystems on Earth. Virology is the study of viruses; it is a branch of microbiology. It is uncertain where viruses first appeared in the evolutionary history of life. Some viruses may have descended from bacteria, while others may have originated from plasmids, which are DNA fragments that can migrate between cells. In the process of evolution, viruses play a key role in horizontal gene transfer, which boosts genetic variety similarly to sexual reproduction. Some biologists believe that viruses are living forms because they carry genetic material, reproduce, and change via natural selection, despite the fact that they lack essential traits, including cell structure, that are typically thought to be crucial requirements for defining life. The term "organisms at the edge of life" and "replicators" have been used to characterise viruses since they share some but not all of these characteristics.