I agree with you!
Let me start with that first and foremost and then add a bit more to my mea culpa.
The statement you quote is from a footnote buried in the first edition of The Complete Guide to Yin Yoga. One of the horrors of being a writer is that your written words are static and do not change, even though you, as the author, continue to learn more as time goes on. This is life in general, I suppose. I sometimes cringe to think of what I taught students in the 1990s when I first became a yoga teacher. Years later, after more education and experience, I can only hope all those students survived!
In my latest edition of The Complete Guide to Yin Yoga, released in 2019, that statement, and many others, has been deleted. In the second edition I took far more care in sourcing studies to back up my assertions, and where I could not be convinced credible evidence existed, I deleted the assertions. (Thus you will find all the first edition quotes from BKS Iyengar are highly caveated in the second edition. His benefits for postures were anecdotal at best.)
The original source of the cancer statement was from a friend who also happened to be a doctor who offered treatment for cancer patients. He was the one who told me of a study that he had just read saying that cancer patients had fewer colds before they acquired cancer than people who were cancer free. I took him at his word.
While it is true that our immune system doesn’t combat cancer once the tumours are established, that is the problem! Cancer is very good at hiding from the immune system, but it is also true that our immune system is constantly removing cancerous cells from our body
. This is called the “elimination phase” where our immune system does keep nascent tumours under control. Sometimes the tumour and our immune system come into a sort of balance where the cancer is not destroyed but neither does it run rampant. This is the "equilibrium phase". However, once the tumour’s growth outstrips the immune systems ability to control it, due usually to mutations that allows the cancers cells to hide from the immune system, we are in the "escape phase" and now the immune system is helpless to combat the cancer. (However, there is some interesting research under way to retrain the immune system to recognize the cancer again and thus eliminate it. This new approach is called immunotherapy. This is the area my friend is now working on. It is not ready for prime time, unfortunately, and lots of work is still needed.)
In any case, when it came time to write the second edition of the book, I wanted to footnote the exact study the good doctor told me about. I could not find it and he could no longer remember it. So, I deleted the statement entirely, just as you suggest. It is not that I no longer believe that there is no role for a healthy immune system in controlling cancer cells, but if i can’t point to the particular study about gettig colds lowering cancer risk, I can’t state that claim. (Curiously, there is some research showing injecting of the cold virus directly into a bladder cancer tumour can help kill it, but that is not the same thing as i was referring to.)
So, again, mea culpa! I do attempt to find credible sources for all my claims and in this case, I should have asked for the paper first and cited it. Instead, I simple repeated the assertion from a credible person. Thus my deleting the statement in the new edition.
Thanks for letting me know your thoughts.