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Today we know a great deal about viruses, which exist in vast profusion. Many are pathogenic, inducing diseases in man, lower animals, and plants. Many are nonpathogenic. We know that the viruses bear intimate relation to the basic processes of life.
Many different disciplines have contributed to our knowledge, which, in the aggregate, is impressive indeed. This knowledge has had an almost explosive development, dating back, in essence, only to the 1890s. The concept of a virus as a distinct entity rested on evidence that accumulated at first rather slowly. Each new datum had to come to terms with the existing framework of knowledge, ie, had to harmonize somehow with existing concepts. As a result many of the new data remained poorly explained until enough evidence had accumulated to bring about the birth of a new concept. This made investigators realize how sharply this new category of infectious agent differed
King LS. The Virus: A History of the Concept. JAMA. 1977;238(22):2415. doi:10.1001/jama.1977.03280230079041