12 THE PRINCIPLES OF IMMUNOLOGY 



of continuity may be due to trauma or may result from profuse 

 growth of bacteria on the surface with the elaboration of poisonous 

 products which may kill the epithelial cells. The former condition 

 is exemplified in infected wounds and the latter in infection by 

 diphtheria bacilli, streptococci, and fungi, such as produce favus, 

 thrush, and pityriasis. Entrance may be favored by changes in the 

 character of secretions, as, for example, the reduction of acidity of 

 the gastric juice in certain forms of chronic gastritis. The bacteria 

 may be implanted in some site which favors their multiplication, as, 

 for example, in the crypts of the tonsils, in the crevices between 

 unclean teeth and in hair follicles. Multiplication in these situa- 

 tions favors the production of poisonous products which may by 

 destruction of cells serve to interrupt surface continuity. Somewhat 

 similar is the fact that extensive destruction of tissues may provide 

 dead material in which saprophytes may develop, and if this mate- 

 rial is so deep as to be excluded from the access of air, conditions 

 favorable to the development of anaerobes are produced. Infec- 

 tion may be favored by the movement of cells and fluids. For 

 example, although leucocytes may take up bacteria, they do not 

 invariably destroy them, and the migration of such leucocytes may 

 lead to the dissemination of organisms by the subsequent death of 

 the leucocyte. The movement of lymph may favor invasion as is 

 seen not uncommonly in those cases of infections of the hand by 

 streptococcus, wherein the lymph flow carries the organisms so as 

 to set up infections of the lymph-vessels and the lymph-nodes, and 

 even of the blood stream. Gaining access to the blood, the circula- 

 tion of this fluid tissue may deposit bacteria in numerous foci 

 throughout the body. The route of invasion depends somewhat 

 upon the type of organism, those of typhoid fever, dysentery, 

 and cholera, gaining access to the intestinal canal through the 

 mouth. Their implantation upon the skin is of no significance, 

 except that they may thence be transferred to the mouth. The 

 gonococcus produces no lesions of the intestinal canal, but implanted 

 in the genital tract, the eye, or the endocardium leads to serious 

 results. If plague bacilli be inoculated subcutaneously in rats a 

 large percentage of the animals survive, but if implanted in the 

 lower respiratory tract small doses suffice to produce fatal infec- 

 tions. The pneumococcus appears to infect man only through the 

 respiratory tract. This phenomenon probably depends in part upon 

 a local susceptibility to the organisms. 



Types of Infectious Disease. The types of infectious disease 

 are differentiated according to the method of invasion and dissem- 

 ination. An organism may grow locally and produce only local 

 manifestations, as seen in a small abscess. It may grow locally and 

 produce marked general disturbances, as is the case in diphtheria, in 

 which instance, although organisms may enter the blood stream, 

 they are usually confined to some focus, such as the tonsils. They 

 elaborate in that situation poisonous substances which are absorbed 

 and set up general manifestations of intoxication. Certain other 



