<P> 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 and viral hepatitis, evade these immune responses and result in chronic infections . Antibiotics have no effect on viruses, but several antiviral drugs have been developed . </P> <P> The word is from the Latin neuter vīrus referring to poison and other noxious liquids, from' the same Indo - European base as Sanskrit viṣa poison, Avestan vīša poison, ancient Greek ἰός poison', first attested in English in 1398 in John Trevisa's translation of Bartholomeus Anglicus's De Proprietatibus Rerum . Virulent, from Latin virulentus (poisonous), dates to c. 1400 . A meaning of "agent that causes infectious disease" is first recorded in 1728, before the discovery of viruses by Dmitri Ivanovsky in 1892 . The English plural is viruses (sometimes also viri or vira), whereas the Latin word is a mass noun, which has no classically attested plural (vīra is used in Neo-Latin). The adjective viral dates to 1948 . The term virion (plural virions), which dates from 1959, is also used to refer to a single, stable infective viral particle that is released from the cell and is fully capable of infecting other cells of the same type . </P> <P> Louis Pasteur was unable to find a causative agent for rabies and speculated about a pathogen too small to be detected using a microscope . In 1884, the French microbiologist Charles Chamberland invented a filter (known today as the Chamberland filter or the Pasteur - Chamberland filter) with pores smaller than bacteria . Thus, he could pass a solution containing bacteria through the filter and completely remove them . In 1892, the Russian biologist Dmitri Ivanovsky used this filter to study what is now known as the tobacco mosaic virus . His experiments showed that crushed leaf extracts from infected tobacco plants remain infectious after filtration . Ivanovsky suggested the infection might be caused by a toxin produced by bacteria, but did not pursue the idea . At the time it was thought that all infectious agents could be retained by filters and grown on a nutrient medium--this was part of the germ theory of disease . In 1898, the Dutch microbiologist Martinus Beijerinck repeated the experiments and became convinced that the filtered solution contained a new form of infectious agent . He observed that the agent multiplied only in cells that were dividing, but as his experiments did not show that it was made of particles, he called it a contagium vivum fluidum (soluble living germ) and re-introduced the word virus . Beijerinck maintained that viruses were liquid in nature, a theory later discredited by Wendell Stanley, who proved they were particulate . In the same year Friedrich Loeffler and Paul Frosch passed the first animal virus--agent of foot - and - mouth disease (aphthovirus)--through a similar filter . </P> <P> In the early 20th century, the English bacteriologist Frederick Twort discovered a group of viruses that infect bacteria, now called bacteriophages (or commonly phages), and the French - Canadian microbiologist Félix d'Herelle described viruses that, when added to bacteria on an agar plate, would produce areas of dead bacteria . He accurately diluted a suspension of these viruses and discovered that the highest dilutions (lowest virus concentrations), rather than killing all the bacteria, formed discrete areas of dead organisms . Counting these areas and multiplying by the dilution factor allowed him to calculate the number of viruses in the original suspension . Phages were heralded as a potential treatment for diseases such as typhoid and cholera, but their promise was forgotten with the development of penicillin . The study of phages provided insights into the switching on and off of genes, and a useful mechanism for introducing foreign genes into bacteria . </P>

When was the first virus found in both plants and animals
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