The study of tumour viruses is a central area of modern cancer research. Two developments are responsible for this remarkable change. Recent developments in microbial genetics have lead to reinterpretation of the virus concept itself. It turns out that certain types of virus can introduce parts of their own genetic material into a cell without killing it or inhibiting its multiplication. The virus material thus introduced may become actually integrated with the gene material of the recipient cell and behave as a new hereditary factor.
Virus infection can thus lead to a permanent change in some cellular characteristics.
This re-evaluation of the virus concept made it possible to understand how a tumour virus might change the regulated behaviour of normal cells to the malignant proliferation characteristic of cancer cells. In the same period many new viruses capable of inducing malignant tumours in mammals were discovered. In 1981 Gross found a virus that induces leukemia in mice. A few years later he and two women scientists, Stewart and Eddy, isolated a remarkable new virus, called polyoma, capable of inducing an array of tumours in many different mammalian species. Since 1960 more than a dozen new tumour virus types have been isolated.