Try looking deeply into almost any specific condition - let's take heart disease. Cholesterol, for instance - is it bad? Is it necessary? Do you need a certain amount or does the amount not matter so long as you have the correct ratio of fats? Why are some people perfectly healthy with cholesterol numbers off the roof, and others can have atherosclerosis with cholesterol numbers off the charts low? Will reducing cholesterol for that second person help them? Hurt them?

For the record, the treatment plan my cardiologist put me on was explained as "all these things seem correlated with not having heart attacks so we'll just do them all and hope it helps you".