DNA damage occurs every day, but we have certain enzymes to repair this damage. Replicating damaged DNA results in DNA mutations or cell death, but not all DNA mutations lead to cancer. Only DNA mutations that encode for genes that carry out functions such as controlled cell death, cell survival and cell proliferation can lead to cancer.
Certain viral infections can be an alternative route to cancer instead of DNA mutations. Viral RNA often contains genes that promote cell proliferation and inhibit cell cycle checkpoints to increase their virulence, but can have the side effect of promoting cancer. Some viruses (called retroviruses) replicate themselves by injecting their RNA into the host cell and making the host cell change the viral RNA into DNA and then has the host cell incorporate the viral DNA into the host cell’s genome.
Tumour cells need further mutations to survive in the bloodstream and become metastatic. Once tumour cells become metastatic they are very difficult to get rid of.
Mitochondrial dysfunction is likely to be the main cause of cancer since: carcinogens also impair mitochondrial function; mitochondrial and mitochondrial DNA is more vulnerable to nuclear DNA and therefore more likely to be damaged first; mitochondrial dysfunction increases reactive oxygen species, inflammation, genome instability and signals hypoxia. Mitochondrial dysfunction can cause the Warburg effect, where tumour cells preferentially metabolise glucose to lactate for energy, even in the presence of oxygen (aerobic glycolysis), which is advantageous to the tumour cells for several reasons
Another cause of metastasis is from when tumour cells merge with macrophages, which allows the tumour cell to better evade the immune system and infiltrate tissues
Pro-inflammatory cytokines have many effects that promote cancer such as oxidative stress, angiogenesis, cell proliferation and inhibiting cell death and the adaptive immune system. Inflammation is strongly associated with cancer, particularly in the same region as the inflammation. For example: IBD and colorectal cancer, thyroid cancer and Hashimoto’s thyroiditis and H. pylori infection and gastric cancer.
A role of the immune system is to kill cancer cells. Mice lacking certain immune cells can develop spontaneous tumors. When these tumours are transplanted into healthy mice they are rejected by immune system. People with weaker immune systems are more likely to have worse prognosis
Some Strategies for Cancer
This is for informational purposes only and is not meant to diagnose or treat any medical condition.
The Conditions that Promote Cancer
Reduce/manage: mitochondrial dysfunction (see Mitochondrial Dysfunction, mainly the second half); chronic inflammation (see Causes of Inflammation); and immune suppression (which will also help with viral infections).
For immune function the following things should help: vitamin A , vitamin D , zinc , nutrient density in general , reducing/managing stress better    and getting good sleep (at the appropriate time)  
* Vitamin A  and vitamin D    also have other anti-cancer effects and the combination is effective at killing cancer cells . Vitamin K2 works synergistically with A and D , has anti-cancer effects   and in a small trial (n=40) megadoses (45mg/d) of K2 reduced the risk of hepatocellular carcinoma by 80% in women with liver cirrhosis 
The Ketogenic Diet for Cancer
A ketogenic diet (moderate protein, very high fat, very low carb, roughly 15:80:5) denies the tumour cells of glucose and provides ketones which can be used fuel by many tissues but usually not by tumour cells. Ketogenic diets help to retain lean mass with cancer cachexia. Ketogenic diets decrease inflammation  and increase AMPK , which: stimulates mitochondrial biogenesis ; increases DNA repair enzymes (!) ; lowers IGF-1 and inhibits mTOR; inhibits lipogenesis in tumour cells (they need a lot of lipids for growth); inhibits cell division; increases oxidative stress in tumours ; and inhibits aromatase