The book's central premise is that diets with a large percentage of animal-based protein are conducive to cancer, heart disease, diabetes, multiple sclerosis, and many other diseases. The solution to avoid cancer and disease? Go vegan.
The author reaches this conclusion by using data from the China Study to compare the "affluent" diets of western countries (US and Europe) with those of poorer countries (China), which are primarily plant-based. The China Study was a large research effort conducted over twenty years in which the author was principally involved.
This is all well and good, but unfortunately, the book fails to establish causality between disease and diet. Epidemiology is a complex and difficult discipline, and if preventing disease were as easy as simply not eating animal protein, it stands to reason that medical science would have figured this out by now. Indeed, by his own admission, the author is an outcast from the conventional medical establishment, a fact that should raise alarms in the reader's head. Additionally, at times the book's tone takes on an almost religious fanaticism as the author rails against animal-based protein. By the final pages, it seems that Campbell's faith in the vegan lifestyle has tainted his view of the China Study data.
As some critical web sites detail, the China Study makes no statistically significant correlation whatsoever between diet and disease. (See Blog of Brad from an organic farmer, and Beyond Vegetarianism, a pro-vegetarian web site that disagrees with Campbell).
Clearly the author has found religion, but until his theories are tested through double blind clinical trials, the establishment is wise to remain skeptical of the claims in this book.