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Virus
A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism.[1] Viruses infect all types of life forms, from animals and plants to microorganisms, including bacteria and archaea.[2] 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,[3] more than 6,000 virus species have been described in detail[4] of the millions of types of viruses in the environment.[5] Viruses are found in almost every ecosystem on Earth and are the most numerous type of biological entity.[6][7] The study of viruses is known as virology, a subspeciality of microbiology.
When infected, a host cell is forced to rapidly produce thousands of identical copies of the original virus. When not inside an infected cell or in the process of infecting a cell, viruses exist in the form of independent particles, or virions, consisting of: (i) the genetic material, i.e., long molecules of DNA or RNA that encode the structure of the proteins by which the virus acts; (ii) a protein coat, the capsid, which surrounds and protects the genetic material; and in some cases (iii) an outside envelope of lipids. The shapes of these virus particles range from simple helical and icosahedral forms to more complex structures. Most virus species have virions too small to be seen with an optical microscope, as they are one-hundredth the size of most bacteria.
https://en.wikipedia.org/wiki/Virus
virus [viยดrus]
any member of a unique class of infectious agents, which were originally distinguished by their smallness (hence, they were described as โfiltrableโ because of their ability to pass through fine ceramic filters that blocked all cells, including bacteria) and their inability to replicate outside of and without assistance of a living host cell. Because these properties are shared by certain bacteria (rickettsiae, chlamydiae), viruses are now characterized by their simple organization and their unique mode of replication. A virus consists of genetic material, which may be either DNA or RNA, and is surrounded by a protein coat and, in some viruses, by a membranous envelope.
Unlike cellular organisms, viruses do not contain all the biochemical mechanisms for their own replication; they replicate by using the biochemical mechanisms of a host cell to synthesize and assemble their separate components. (Some do contain or produce essential enzymes when there is no cellular enzyme that will serve.) When a complete virus particle (virion) comes in contact with a host cell, only the viral nucleic acid and, in some viruses, a few enzymes are injected into the host cell.
Within the host cell the genetic material of a DNA virus is replicated and transcribed into messenger RNA by host cell enzymes, and proteins coded for by viral genes are synthesized by host cell ribosomes. These are the proteins that form the capsid (protein coat); there may also be a few enzymes or regulatory proteins involved in assembling the capsid around newly synthesized viral nucleic acid, in controlling the biochemical mechanisms of the host cell, and in lysing the host cell when new virions have been assembled. Some of these may already have been present within the initial virus, and others may be coded for by the viral genome for production within the host cell.
https://medical-dictionary.thefreedictionary.com/virus