non-cellular, submicroscopic infectious agent that replicates only inside the living cells of an organism

A virus is a small infectious agent that replicates only inside the living cells of other organisms. Viruses can infect all types of life forms, from animals and plants to microorganisms, including bacteria and archaea.


  • A virus is a form of life with very simple requirements. The basic needs of a virus are a nucleic acid to be transmitted from generation to generation (the genome) and a messenger RNA to direct the synthesis of viral proteins. The critical viral proteins that the messenger RNA must encode are those that coat the genome and those that help replicate the genome. One of the great surprises of modern virology has been the discovery of the variety of genetic systems that viruses have evolved to satisfy their needs. Among the animal viruses, at least 6 totally different solutions to the basic requirements of a virus have been found.
  • In 1910 I described a malignant chicken sarcoma which could be propagated by transplanting its cells, these multiplying in their new hosts and forming new tumors of the same sort. In other ways the growth showed itself to be a neoplasm of a classical sort, yet, as reported in 1911, its cells yielded a causative virus. Numerous workers had already tried by then to get extraneous causes from transplanted mouse and rat tumors but the transferred cells had held their secret close. Hence the findings with the sarcoma were met with down-right disbelief, though soon several other, morphologically different, “spontaneous” chicken tumors were propagated by transplantation and from each a virus was got causing growths of its kind. Not until after some 15 years of disputation amongst oncologists were the findings with chickens deemed valid, and then they were relegated to a category distinct from that of mammals because from them no viruses could be obtained. Only in 1925, through the efforts of a British worker, W.E. Gye, was much attention given them by scientists.
  • Why has it been so difficult to identify infectious agents as cancer-inducing factors in humans? Because there is no human pathogenic infectious agent causing cancer as the acute consequence of infection ... Infections linked to human cancers are common in human populations, most of them were present during the whole human evolution,and only a small proportion of infected individuals develops the respective cancer type ... Except for rare germline mutations, (XLLP), cancers linked to infection commonly occur decades after primary infection ...

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