These microscopic organisms do not by their mere presence set up disease, unless indeed they are in such overwhelming numbers as to block the capillary blood vessels mechanically. Some of them are carried broadcast in the blood current, while others remain at the point of entrance; in either case they elaborate certain products, termed toxines, which act, either locally or through the circulation, to cause the disease. These toxines eventually kill the micro-organisms that produced them, quite as an animal may be smothered in its own exhalations; or at least they would do so if the "host" survived long enough for the completion of the process. Meantime, they have either killed the "host" or been defeated by certain very interesting natural processes. But before either of these occurrences has had time to take place, fortunately, in the great majority of instances, save those of exposure to the most deadly of infections, the vital power of the invaded individual has coped successfully with the invaders at the very point of attack--has repulsed the attacking party without appreciable impairment of its own force--and no illness results. For example, practically all of us inhale the germ of consumption repeatedly, but most of us suffer no harm from it simply because the fluids which bathe the surface on which the germ effects a lodgment are endowed with properties which either kill the germ or rob it of its power for harm; but these properties suffice only when the general health is unimpaired.