What is the evidence that supports chronic and severe depression and stress increasing ones risk of developing cancer?
National Cancer. Institute info sheet shows weak support. REF: https://www.cancer.gov/about-cancer/coping/feelings/stress-fact-sheet.
The risk is low. As i discuss in my book, "a cancer prevention guide for the human race, " breast cancer risk may be slightly increased with chronic depression, but the evidence for this is rather weak (and evidence that chronic depression and stress causes other cancers is even weaker). People who engage in activities known to increase cancer risk, because they're stressed, can increase their cancer risk, though.
Not much. It doesn't make sense biologically. Cancer is induced by things that cause damage to DNA molecules, promoted by things that cause cell death and replacement. "Studies" are weak at best and having occult cancer itself can cause depression. If I do 20 "studies" of whether "A causes B", odds are that one will show A causes B, and one will show A prevents B. Ignore this stuff & live a good life.