A virus is a small infectious agent that replicates only inside the living cells of an organism. Viruses can infect all types of life forms, from animals and plants to microorganisms, including bacteria and 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, about 5,000 virus species have been described in detail, of the millions of types of viruses in the environment. Viruses are found in almost every ecosystem on Earth and are the most numerous type of biological entity. The study of viruses is known as virology, a sub-speciality of microbiology.
While 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, about 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 move 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 to be a life form, because they carry genetic material, reproduce, and evolve through natural selection, although they lack key characteristics (such as cell structure) that are generally considered necessary to count as life. Because they possess some but not all such qualities, viruses have been described as "organisms at the edge of life", and as replicators.Viruses spread in many ways. One transmission pathway is through disease-bearing organisms known 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. Influenza viruses are spread by coughing and sneezing. Norovirus and rotavirus, common causes of viral gastroenteritis, are transmitted by the faecal–oral route, passed by contact and entering the body in food or water. 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 usually 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 AIDS, HPV infection, and viral hepatitis, evade these immune responses and result in chronic infections. Several antiviral drugs have been developed.

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  1. EHJ

    Now a Rabbit Virus is here. Awesome.
  2. W

    Governor Inslee's virus lockdown and state budget crash - Interview with Rep. Jim Walsh

    Broad cuts to the budget could include cutting Evergreen State College. WDFW may see cuts that lead to efficiency. ~Whitney
  3. gmerkt

    Catching up on projects during the virus thing - and in good weather

    So long as we've got the isolation from the virus thing going on, might as well catch up on some chores. Especially since the weather has been nice nearly all of April so far. One project that I've been putting off for too long was new hydraulic brake hoses on the front disc brakes of my 72...
  4. RicInOR

    Earthquakes, Volcanoes, Locusts, Hurricanes, Tornadoes, Virus' ...

    I gotta say, 2020 is giving us a lot for our money.
  5. No_Regerts

    Corona Virus Playlist

    Whats on your quarantine playlist? I had a few ideas if Time Life wanted to make a collection: 1.) Georgia Satellites: Keep Your Hands To Yourself 2.) Celine Dion: All By Myself 3.) Drowning Pool: Bodies 4.) MC Hammer: Can’t Touch This 6.) Bone Thugz ‘n’ Harmony: Crossroads 7.) Bon Jovi: Bad...
  6. conditor22

    Hospitals not yet overwhelmed by rush of virus patients

    Hospitals not yet overwhelmed by rush of virus patients
  7. jim97701

    OMG now the ocean will give you corona virus Only in California
  8. Legs

    Japan has escaped Corona virus so far...... Japan seemed like such a powerhouse 20 years ago, buying up properties and companies here in the USA. They have managed to avoid the corona virus epidemic so far, but that may all be ending. Another densely populated...
  9. ATCclears

    The coming age of Dispertion

    5-10 years from now people many will forget, but some may not and choose a bucolic lifestyle where they telecommute or find other forms of work.
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