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. Cancer caused by irradiation is uncommon in domestic animals except those individuals without pigmentation in their skin. Lifetime exposure to the sun can cause cancer on the white ears or white face of dogs. Inherited cancer is somewhat different. One copy of the gene comes from the dam and one copy of the gene comes from the sire.