Disease has been variously regarded as an entity, a process, a condition. It has been mentioned in terms which would almost personalize it, such as, “attacked by pneumonia,” “seized with cramps,” “in the clutches of tuberculosis.” Men have endeavored constantly to discriminate between diseases and to learn the appearance and peculiarity of each, and have resolved each into its peculiar elements only to learn that the merging lines between two diseases or between cases of the same disease are imperceptible. It is no more possible to define any one disease within exact limits and to distinguish it from all others than to consider one function of the human body without studying its interdependence with others. Disease is a _process_. It is a natural process. It follows certain well-defined laws and consists in the abnormal performance of function in some bodily organ or organs, or in the untimely performance of some function which would be normal in its proper chronological relation with other functions or at another period of the body’s development. The balance of function of the body is destroyed--some function intensified or diminished--that is all. Every disease, properly studied, reveals its functional base. Disturbances of the functions of _growth_, _nutrition_, and _repair_ produce changes in structure, physical evidences of disease. It is probable that every disease has a certain amount of structural change connected with it; it is hard to conceive of functional derangement without structural change, in a universe in which Nature is eternally building, destroying, or modifying organic peculiarities to meet changing functional demands. But in many instances this structural change is so slight as to be undiscoverable; such diseases are called “functional” to distinguish them from those in which structural pathologic changes are directly discernible, called “organic.” Beginning of the Process Recognizing the fact that disease consists in a succession of steps or a series of events, each depending upon the next preceding one and making possible its successor, and desiring to arrest or check this process and correct the damage done, in other words, “to cure disease,” the question arises, “Where does this process begin?” If we wish merely to check the process or to modify it, as does medicine, the etiology of the disease is less important than the present state. It is then more important that we understand the changes which are taking place in the body at the time of our attempt, the condition of each organ at that time, and the general recuperative or resisting power of the individual.