This paper briefly retrospects the course of evolution of the collateral disease doctrine （CDD） in the past several thousands of years

discusses the causes and pathogenesis of collateral diseases

and tries to explain the pathological and physiological bases of the diseases from the angle of Western medicine

in order to more deeply understand the CDD

fully comprehend its significance in both theory and clinical practice and to further enhance TCM clinical efficacy in treating difficult and complicated cases of diseases.