CHAPTER III 



THE OFFENSIVE FORCES OF THE INVADING 

 MICROORGANISM 



WHILE the protection which the macroorganism is afforded by its 

 epithelial covering is thus undoubtedly of great importance, it is a 

 striking fact that infections of serious extent are after all compara- 

 tively rare, if we consider the frequency with which injury of the 

 tegumentary and mucous surfaces occur. Minor infections, on the 

 other hand, are common enough, and the question naturally arises: 

 Why does not every infection become generalized and lead to the 

 destruction of the host? Evidently this must depend upon one 

 of two factors (sc., an interaction between the two), viz., the nature 

 of the microorganism and the resistance which the macroorganism 

 offers to the presence of the other. Collectively those forces which 

 are at the disposal of the invading organism, and in virtue of which it 

 strives to maintain itself in its new environment, may be termed its 

 aggressive forces, in contradistinction to the defensive forces of the 

 host. The former will occupy our attention in the present chapter. 



Necroparasites. Bacteriological examination of the blood during 

 the life of the patient and of the various tissues after death reveals 

 a remarkable difference in infections with different organisms. We 

 thus find that certain bacteria, such as the diphtheria bacillus, the 

 tetanus bacillus, and the bacillus botulinus, are possessed of a very 

 low grade of infectiousness, if by this term we mean their power 

 of multiplying in the invaded organism. The infection is almost 

 always strictly local during the life of the patient ; a general infection 

 is indeed exceedingly rare, and when it occurs it does so only sub 

 finem mice or after the death of the patient. The tetanus bacillus 

 particularly is practically unable to maintain itself in normal living 

 tissues, and in cases of infection owes its limited development either 

 to the damage done by an associated infecting agent or by direct 

 mechanical injury. Even so, the organism has frequently disappeared 



