Before disease can attack, and develop itself in the bodies of men or animals, the existing equilibrium of the vital powers must be disturbed; and the most common causes of this disturbance we have already alluded to. In reference to the milk, butter, cheese, &c., of infected animals, and their adaptation to develop disease in man, and in other locations than those referred to, we observe, that when a quantity, however small, of contagious matter is introduced into the stomach, if its antiseptic properties are the least deranged, the original disease (milk sickness) is produced, just as a small quantity of yeast will ferment a whole loaf. The transformation takes place through the medium of the blood, and produces a body identical with, or similar to, the exciting or contagious matter. The quantity of the latter must constantly augment; for the state of change or decomposition which affects one particle of the blood is imparted to others. The time necessary to accomplish it, however, depends on the amount of vital resistance, and of course varies in different animals. In process of time, the whole body becomes affected, and in like manner it is communicated to other individuals; and this may take place by simply respiring the carbonic acid gas, or morbific materials from the lungs, of diseased animals in the infected districts. We are told that the latent condition of the disease may be discovered by subjecting the suspected animal to a violent degree of exercise. This is a precaution practised by butchers before slaughtering animals in any wise suspected of the poisonous contamination;[29] for according to the intensity of the existing cause, or its dominion over the vital power, it will be seized with tremors, spasms, convulsions, or even death. The reader is, probably, aware that an excess of motion will sometimes cause instant death; for both men and animals, supposed to be in excellent health, are known to die suddenly from excessive labor. In some cases of excess of muscular exertion, the active force in living parts may be entirely destroyed in producing these violent mechanical results: hence we have a loss of equilibrium between voluntary and involuntary motion, and there is not sufficient vitality left to carry on the latter. Professor Liebig says, "A stag may be hunted to death. The condition of metamorphosis into which it has been brought by an enormous consumption both of force and of oxygen continues when all phenomena of motion have ceased, and the flesh becomes uneatable." A perfect equilibrium, therefore, between the consumption of vital force for the supply of waste, protecting the system from encroachments, and for mechanical effects, must exist; the animal is then in health: the contrary is obvious.