Certain bacteria, as those of diphtheria and tetanus, are possessed of a very low grade of infectiousness, by which is meant their power of multiplying in the invaded body. The infection is almost always strictly local during the life of the individual; a general infection being exceedingly rare, and occurring as a terminal or a post-mortem condition. The tetanus bacillus, particularly, is practically unable to maintain itself in normal living tissues. In cases of infection it owes its limited development either to the damage done by an associated infecting agent or by direct mechanical injury. Even then the organism has frequently disappeared from the body entirely at the time when the patient is actually dying from the effects of its brief sojourn. Evidently, its aggressive powers are minimal and, even though it kills through its highly poisonous toxin, the resistance which the animal body offers to its presence is entirely sufficient to prevent its active development. Similar conditions exist in regard to the diphtheria bacillus. It is questionable whether it can gain access to the deeper tissues through intact superficial structures, but by means of its own toxin it is evidently capable of causing marked destruction after that superficial barrier has been passed. The above instances show that the infectious and toxic properties of an organism are two independent factors, which in the case of tetanus and diphtheria bear an inverse relation to each other.

An altogether different behavior is seen in a group of organisms represented by the anthrax bacillus and that of chicken-cholera. In these the local infection is followed almost immediately by a generalized infection, the organisms not only living, but actually multiplying freely in the body of the host. Their aggressivity, as compared with the previously mentioned type, is greatly developed, while their toxicity is practically nothing.

Between these two types already mentioned stand the cholera vibrio and the typhoid bacillus. Their aggressivity is quite well developed, particularly that of the typhoid bacillus, which is commonly present in the blood and tissues. In addition to their aggressiveness, the organisms of this class possess a well-marked toxicity, the effect of this appearing quite early in the course of the infection, and leading to a fairly characteristic clinical picture of the corresponding infectious disease.

Generally speaking, it may be said that the ability of microorganisms to do harm depends upon the injurious nature of the substances they can produce. There are probably three groups of substances that are now recognized as of importance in connection with the clinical picture of the infectious diseases. They are:

1. True toxins or exotoxins, extracellular and soluble.

2. Endotoxins, intracellular and insoluble.

3. Bacterial proteins.