Each day of your dog's life all of the genes of each cell are subjected to a barrage of insults that have the potential to cause a mutation. One category of toxic chemicals comes from within the cell. These toxic chemicals are normal products of daily chemical activities within each cell. For the most part, cells have developed mechanisms for detoxifying these waste products. Another category of toxic chemicals are components of the normal diet.
Every diet contains plant matter.
The chemicals that are toxic to the natural predators of plants are also toxic to the living cells of your dog in many instances. A third category of toxic chemicals is the man-made chemicals. There is a long list of man-made chemicals that are known to cause mutations in the DNA of cells and result in cancer. Viruses can cause a mutation in a gene. However, there are very few viruses that have been proven to cause cancer in dogs, and at this time it does not appear that viruses are a significant cause of cancer in dogs.