There are many reasons for the onset and progression of disease. In clinical practice, diseases are routinely classified into primary and secondary diseases. For example, the onset or development of a lesion due to external factors or external stimuli is called a secondary disease. On the contrary, diseases that appear without a clear cause or external interference are called primary diseases. In clinical practice, the most common term for diseases with an unknown cause or for which no clear cause has been studied is primary disease. Diseases with obvious triggering factors and for which the developmental process is correctly understood are called secondary diseases.