Any human organism is home to viruses and bacteria. However, viruses (as well as other intracellular parasites) are interested in continuous division of the host cells. Unlimited division of the host cells means unlimited expansion of the living space and possibility for unlimited multiplication of viral particles. For this, human Cell Cycle Regulation System has to be affected in such a way to induce unlimited division of host cells. The present work describes oncogenome`s model of cancer development, according to which, in order to stimulate cell division, viruses affect a gene (or several genes) during transduction of a signal for cell division from growth factor to cyclin/CDK system. At the same time, controlling gene (p53, RB etc.) function and a switch to apoptosis are suppressed. Cell switch to cell division implies inability of the cell to carry out the functions of a differentiated cell. Spreading of viruses (bacteria) to the adjacent cells and forcing them into division, with simultaneous loss of differentiated cell functions, will become evident as the formation of malignant tumor or cancer.