One major reason is that these diseases--ulcers, high blood pressure, heart and arterial disease, and so on--are not acute illnesses brought on by a single cause, as many past epidemic diseases were, but are chronic illnesses brought on by numerous factors, so it is relatively hard to clarify cause and effect relationships.
原因之一是，這些疾病，如癌症、心臟血管疾病、胃瘍……，都不似過去是由單一因子引起的急症，而是多重因素造成的慢性病，因果關係較難證明。