Virus
A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect any life forms, from animals as well as plants to microorganisms, including bacteria as well as archaea. 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, more than 9,000 virus species pretend been pointed in unit of the millions of nature of viruses in the environment. Viruses are found in near every ecosystem on Earth and are the almost numerous type of biological entity. The explore of viruses is requested as virology, a subspeciality of microbiology.
When infected, a host cell is often forced to rapidly create thousands of copies of the original virus. When not inside an infected cell or in the process of infecting a cell, viruses equal in the form of self-employed person particles, or virions, consisting of i the genetic material, i.e., long molecules of DNA or RNA that encode the formation 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 external envelope of lipids. The shapes of these virus particles range from simple helical and icosahedral forms to more complex structures. Most virus types have virions too small to be seen with an optical microscope and are one-hundredth the size of most bacteria.
The origins of viruses in the evolutionary history of life are unclear: some may have evolved from plasmids—pieces of DNA that can advance between cells—while others may have evolved from bacteria. In evolution, viruses are an important means of horizontal gene transfer, which increases genetic diversity in a way analogous to sexual reproduction. Viruses are considered by some biologists to be a life form, because they carry genetic material, reproduce, and evolve through natural selection, although they lack the key characteristics, such(a) as cell structure, that are broadly considered essential criteria for determining life. Because they possess some but not all such qualities, viruses have been intended as "organisms at the edge of life", and as replicators.
Viruses spread in numerous ways. One transmission pathway is through disease-bearing organisms so-called as vectors: for example, viruses are often transmitted from plant to plant by insects that feed on plant sap, such as aphids; and viruses in animals can be carried by blood-sucking insects. many viruses, including influenza viruses, SARS-CoV-2, chickenpox, smallpox, and measles, spread in the air by coughing and sneezing. Norovirus and rotavirus, common causes of viral gastroenteritis, are transmitted by the faecal–oral route, passed by hand-to-mouth contact or in food or water. The infectious dose of norovirus required to produce infection in humans is fewer than 100 particles. HIV is one of several viruses transmitted through sexual contact and by exposure to infected blood. The variety of host cells that a virus can infect is called its "host range". This can be narrow, meaning a virus is capable of infecting few species, or broad, meaning it is capable of infecting many.
Viral infections in animals provoke an immune response that normally eliminates the infecting virus. Immune responses can also be produced by vaccines, which confer an artificially acquired immunity to the specific viral infection. Some viruses, including those that cause HIV/AIDS, HPV infection, and viral hepatitis, evade these immune responses and sum in chronic infections. Several a collection of matters sharing a common features of antiviral drugs have been developed.