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Everybody has genes for cancer in every cell of their body - these are called 'oncogenes.' For most of us, the start codon - a specific three sequence base pair, is missing. Point mutations (single base pair mutations) normally occur at a very low frequency, and are further reduced by the like of antioxidants. Inversely, carcinogens are substances we know to cause point mutations to occur more frequently, as does stress. Now if upstream from an oncogene you already have two of the three base pairs to be a start codon and a point mutation causes the third base pair to turn the sequence into a start codon you now have cancer. Each time that cell divides, unless there just happens to be a mutation in each of the daughter cells oncogene start codons (very unlikely), each daughter cell will likewise have an active gene for cancer.

As mentioned above, stress can lead to a higher incidence of point mutations. I think it is fairly safe to say these days, with the global economy and various conflicts going on, are the most stressful we've seen in recent history.

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