There is a widespread (and correct) recognition that a well-nourishing diet is the best way to prevent disease, but it is usually qualified with the (often incorrect) belief that medicine is the best treatment for disease.
Here, quoted by Asclepius at Natural Messiah, is T. Colin Campbell, the misguided advocate of vegan dieting:
I think the institution of science, which has basically served a very reductionist way of thinking, that is producing little pills and magic bottles to do this that and everything else, that’s what medical science has largely been, been fostering, been concerned with, and interested in.
And of course it … serves our sense of how to control disease through cure, but, it doesn’t serve the public. Prevention is really the way to go, and at the centre of the plate for prevention is nutrition, how we decide to eat and how we decide to behave otherwise, and that’s a very comprehensive sort of lifestyle dietary change. That’s where we get good health – that’s what the public needs to know, and science is not delivering it. 
This is true as far as it goes: Medicine is overly preoccupied with drugs, which rarely cure and often do nearly as much harm as good. Nourishing diets really do prevent disease and deliver good health.
However, stopping there risks leaving sick people with the impression that, having missed their chance at prevention, they must now “control disease through cure” – in other words, that they have to look to doctors and drugs, not to a nourishing diet, for healing. This is, I think, usually a mistake.
We believe that most diseases are the cumulative effect of food toxins, malnutrition, and infection. Food toxins and malnutrition sabotage immune defenses and enable infections to proliferate and damage the body. A well-nourishing diet enables the immune system to control infections and maintain good health.
When a cure is needed, repairing the diet must still be among the highest priorities. A poor diet will continue to disable the immune system and prevent recovery. Repairing the diet will rejuvenate the immune system and give the patient a fighting chance.
Medicine is not a cure-all. Drugs are not cure-alls. They can aid the immune system, but it is the immune system which must ultimately defeat pathogens and heal injured tissues. The source of a cure is within.
Nutrition has been largely disjoined from therapy. Medicine today fails to utilize diet as an adjunct to therapy. As long as that continues, medicine will continue to be ineffective against chronic disease.
 T. Colin Campbell, interviewed by One-Off Productions, 1997, http://www.mcspotlight.org/people/interviews/campbell.html.