But the methods of combating the different diseases when once contracted in the human body, based on the knowledge obtained of the life history of these germs, have been the most important result of their biological study. A large part of this knowledge has been acquired by the study of animals which have been found susceptible and so available for experimental investigation, and it may be that the impossibility of studying measles, for instance, in animals, may be one reason why the germ has never been discovered. There is no evidence that animals suffer spontaneously from such diseases as typhoid fever, Asiatic cholera, leprosy, yellow fever, smallpox, measles, and so on; but it seems that in animals, as in man, the disease is the direct result of the life and growth in the animal of the characteristic disease-producing germ. The fact that diphtheria or tuberculosis can be experimentally given to rabbits or guinea pigs is without doubt the chief source of our knowledge of those diseases, although, in general, it is impossible to produce diseases in any animal which will be, clinically, precisely like the disease as it appears in man. The converse of this is also true, namely, that when it has been found impossible to experimentally inoculate an animal with a disease supposed to be bacterial in nature, then but very little of that disease is known. The most important result of bacterial studies has been the production of what are known as antitoxins, and no more wonderful discovery has ever been made. To understand as best we may the principle involved, it is necessary to explain the process of bacterial attack. When bacteria capable of producing disease are introduced into the system, either through the mouth or into the lungs or into the blood through some skin abrasion, the bacteria, finding there a congenial habitat, thrive, grow, and multiply. In some cases, this bacterial growth results only in breaking down the cell tissues at the point or in the vicinity of the place where growth occurs; for instance, if a cut is made with a dirty knife, that is, one carrying bacteria on the blade, and is not immediately washed out with an antiseptic solution, bacteria will grow and pus will form in the cut. Similarly, a splinter, if not removed and cleansed, will produce a pus-forming wound. But unless a very extensive suppuration starts, the difficulty is all local. So it is with consumption, when the bacteria are localized in the lungs and by their growth destroy the lung tissue without, at least for many weeks, affecting the general health.