Health, using the word in a definite sense, is a theoretical condition; it implies perfect correspondence of the organism with the surrounding conditions, and its maintenance includes perfect adaptation of any changes which may occur in either or both. What Mr. Herbert Spencer says of life may be paraphrased so as to apply accurately to conditions of health. Perfect correspondence would be perfect health. Were there no changes in the environment but such as the organism had adapted changes to meet, and were it never to fail in the efficiency with which it met them, there would be unimpaired and perpetual health. That these conditions do not exist in nature is perfectly obvious, and in speaking of health various and modifying terms are used to indicate degrees of health without suggesting the actual existence of disease. If there were an absolute standard of health there would be no difficulty in defining disease; but in reality there is no distinct boundary line, and the transition from health to disease may be so gradual that it might not be possible to say where the one ends and the other begins.

Disease may be taken in a general sense to mean any disturbance of the structures or functions of a living being. The derangement may be acute when it is severe and rapid in its progress; chronic when it assumes a lower type and is disposed to continue; sporadic when it is the result of ordinary causes arising from without, as exposure to climatic changes, insufficient or impure food; epizootic when it extends to a large number of animals at the same time as the result of some cause which is generally distributed; enzootic when it affects a number of animals in a particular locality owing to local conditions; and recurrent when it exhibits a tendency to return after the affected animals have apparently recovered.

The science of pathology teaches that the strict meaning of the term disease, or loss of ease, cannot consistently be retained in reference to many morbid conditions because they do not necessarily produce any discomfort, and can only be considered as disease for the reason that they are a departure from the ordinary normal or healthy condition of the structures or functions. Certain forms of bony tumours which occur in situations where they do not interfere with the mechanism of the skeleton, and are unattended with pain or inconvenience, may be referred to in illustration of this proposition.

Pathology includes everything connected with a departure from health, and implies, therefore, a wide range of knowledge in regard to all the circumstances under which structural or functional changes are developed. Etiology is a division of the science of pathology which relates to the causes of disease. Semiology refers to symptoms or indications, or, in other words, to the external expressions of a morbid condition. Predisposing causes are those which, as the term implies, act injuriously upon the organism and render it liable to the influence of more energetic causes. Various circumstances of an ordinary kind, such as changes of temperature, exertion, quality and quantity of food, the impurities in the atmosphere, age, sex, conformation, temperament, and hereditary disposition may all be classed as predisposing causes of disease. Nosology is the classification of disease. Diagnosis implies the accurate definition of a disease, its position, nature, and localization. Prognosis relates to the probable termination of disease, or the expression of the opinion of the observer based upon his diagnosis; its value necessarily depends upon his experience of the course which the disease has taken in similar cases, or upon the accuracy of his judgment in regard to the actual moibid changes in the structures or functions in the particular case under consideration.