“Settlements of this kind may, however, become of importance, if the bacteria proceed to develop to any unusual extent. This happens when the contents of the natural cavities in question remains unchanged for any great length of time, or when (as in catarrh) the normal secretion undergoes some alteration. The products of bacterial fermentation may then accumulate to an excessive amount, and products may be found which do not normally occur. Highly poisonous substances are formed in many of the bacterial decompositions. One of the most speedily fatal diseases, septicæmia, is due to blood-poisoning of the system with the products of bacterial putrefaction, or sepsis. “Putrid or septic poison may be absorbed by wounds as well as by mucous surfaces. Septicæmia, which has just been cited as an instance of septic poisoning, is generally due to wound infection. It is due to the absorption of products of bacterial decomposition formed in a wound contaminated by bacteria. “_Infectious bacteria_ have the power of settling, not merely in the ingesta and secretions or in dead tissue, but also in living tissue. This happens chiefly in the mucous membrane of the lungs. The uninjured skin is protected against invasion by the horny epidermis. Many bacteria can settle in perfectly healthy mucous membrane. In the case of others we must imagine that they do not find a proper soil for their development, unless the mucous membrane is injured or altered. Of course, injury or alteration of this kind may serve to make the outer skin or any other accessible tissue, the starting-point of a bacterial invasion (wound infection). All that is necessary is that a bacterium should reach a spot that affords the conditions for its development. If this occurs, it multiplies and forms colonies or swarms. These may, according to the species of the fungus and the nature of the soil, remain in aggregation, forming heaps or masses, or may spread through the tissues. In general terms we may say that local settlements of bacteria will sooner or later bring about degeneration and necrosis of the affected tissue. When this may occur, and how widely it may spread, are circumstances depending on the nature of the bacteria and of the tissue. “The inflammatory processes set up by bacterial action may be of very different intensity and extent in different cases. It may be slight or transient, or may be severe and issue in suppuration and an abscess.” The above quotation is perhaps as concise and complete an explanation as the space in this article will permit, and if thoughtfully considered, it will be the means of understanding what is to be said of the disease under consideration.