Risk factors anything in our thinking, behavior, body or environment that increases the likelihood of illness are opposed by "resistance resources" we possess. These may include an optimistic way of looking at life, a good social support system, and a strong set of genes. The balance between risk factors and resistance resources largely determines whether we get sick or stay healthy.

How much risk a factor may contribute to our getting sick has been established for various diseases? For instance, cigarette smoking doubles our risk of developing coronary heart disease. High serum cholesterol, hypertension and Type A behavior have each been found to contribute about the same amount of risk for CHD as does smoking. When two of these factors are present, the risk is four times greater; when we have three risk factors, the risk is eightfold higher.

Although heart disease and all other leading causes of death and disability today require the presence of multiple risk factors, there is still "mounting pressure to explain the appearance of most chronic diseases on the basis of such single variables as cigarette smoking, alcohol consumption or exposure to other hazardous substances." Smoking, drinking and hazardous substances by themselves do not "cause" disease, although "this assumption is widely held despite the fact that the majority of those thus exposed do not succumb prematurely to the disorder in question."

Fund raising campaigns urge the public to "fight disease" with dollars so that "the cause" and "cure" can be found for everything from cancer to chronic ileitis. Internist Caroline Thomas of Johns Hopkins observes that medicine's notable success in eradicating specific infectious diseases by means of specific agents has led to the general belief that chronic disease can be similarly abolished when the single 'cause' for each disorder is found." But she notes:

“Thirty years of intensive research have so far failed to discover the single "cause" of cancer, heart attack, or mental illness. The time has now come to consider another concept of disease etiology.”