Diphtheria antitoxin, for instance, the first and best known triumph of the new medicine, is the antidotal substance formed in the blood of a horse in response to a succession of increasing doses of the bacilli of diphtheria. Similar antidotal substances are formed in the blood in all other non-fatal cases of infectious diseases, such as typhoid, pneumonia, blood-poisoning, etc.; and the point at which they have accumulated in sufficient amounts to neutralize the poison of the invading germs, forms the crisis, or "turn" of the disease. So that when we speak of a disease "running its course," we mean continuing for such length of time as the body needs to produce anti-bodies in sufficient amounts to check it. The principal obstacle to the securing of antitoxins like that of diphtheria for all our infectious diseases is, that their germs form their poison so slowly that it is difficult to collect it in sufficient amounts to produce a strong concentrated antitoxin in the animal into which it is injected. But the overcoming of this difficulty is probably only a question of time. Obviously, if infectious disease be, as we say, "self-limited," that is to say, if the body will defeat the invaders with its own weapons, on an average in nine cases out of ten, our wisest course, as physicians, is to back up the body in its fight. This we now do in every possible way, by careful feeding, by rest, by bathing, by an abundance of pure water and fresh air, with the gratifying result that we have already reduced the death-rate in most fevers, even such as we have no antitoxin against, or may not even have discovered the causal germ of, to one-half and even three-fourths of their former fatality. The recognition of the fact that disease has a natural history, a birth, a term of natural life and a death, has already turned a hopeless fight in the dark into a victorious campaign in broad daylight. Huxley's pessimistic saying that typhoid was like a fight in the dark between the disease and the patient, and the doctor like a man with a club striking into the melee, sometimes hitting the disease and sometimes the patient, is no longer true since the birth of bacteriology. Nowhere can the natural history of disease be more clearly seen or more advantageously studied than in the case of typhoid fever. The cause of typhoid is simplicity itself, merely drinking the excreta of some one else, "eating dirt," in the popular phrase; simple, but of a deadly effectiveness, and disgracefully common. The demon may be exorcised by an incantation of one sentence: _Keep human excreta out of the drinking water._ This sounds simple, but it is n't. Eternal vigilance is the price of health as well as of liberty.