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The virus is a non-cellular infectious agent that can be reproduced only within living cells. Viruses affect all types of organisms, from plants and animals to bacteria and arches (bacterial viruses are commonly referred to as bacteriophages). Viruses that can replicate only in the presence of other viruses (satellite viruses) are also detected.
Since the publication in 1892 of Dmitry Ivanovsky’s article describing the non-bacterial pathogen of tobacco plants, and the discovery of the tobacco mosaic virus by Martin Beyerin in 1898, more than 6,000 types of viruses have been described in detail, although they are supposed to exist more than a hundred million. Viruses are found in almost every ecosystem on Earth, they are the most numerous biological form. The study of viruses deals with the science of Virology, a section of microbiology.
In animals, viral infections cause an immune response, which most often leads to the destruction of the pathogenic virus. Immune responses can also be triggered by vaccines that give active acquired immunity against a specific viral infection. However, some viruses, including the human immunodeficiency virus and pathogens of viral hepatitis, are able to escape from the immune response, causing chronic illness. Antibiotics do not act on viruses, but several antiviral drugs have been developed.