INFECTION AND INFECTIOUS DISEASE 21 



also staphylococci, streptococci, colon bacilli, and in herbivorous 

 animals the tetanus bacillus and the anthrax bacillus. 



Evidently, then, the mere presence of the disease-producing organ- 

 isms on the tegumentary and mucous surfaces of the body does not 

 indicate either that the individual has passed through the disease 

 in question, or is ill at the time, or is about to fall ill; nor does the 

 mere presence of such organisms constitute infection. If, however, 

 the normal epithelial barrier has once been passed and the deeper 

 structures have been invaded then we can speak of infection, and 

 when infection has once taken place then we may also find clinical 

 evidence of such infection, i. e., symptoms of the corresponding 

 infectious disease; but it does not follow because infection has 

 taken place that symptoms of disease must of necessity follow. 



Infection and infectious disease are thus not synonymous terms. 

 The two may be associated, but they are not necessarily so. Infec- 

 tion probably always results in a disturbance of the normal functions 

 of the host, and if this disturbance rises beyond a certain point 

 symptoms may develop which constitute what clinicians regard 

 as the corresponding infectious disease. If, however, the normal 

 functional equilibrium of the host is but little affected by the pres- 

 ence of the invading organism, no clinical symptoms of disease 

 develop, notwithstanding the fact that the microorganisms may 

 have multiplied in the body to an enormous extent. There would 

 thus be an infection, but no infectious disease, using the term disease 

 in the ordinary sense of the word. In some cases of this kind, as in 

 anthrax, for example, the infection may nevertheless result fatally, 

 but the period of time during which the animal shows clinical symp- 

 toms of infection is so brief that one can hardly speak of evidence 

 of disease; when this appears death is virtually at hand. In other 

 cases, such as some of the protozoan infections of the blood of various 

 animals (ordinary rat trypanosomiasis, for example), no harm seems 

 to result to the host whatever, even though the blood be swarming 

 with the invaders. There is thus infection of a high order without 

 any evidence of associated disease. 



Evidently, then, it is necessary to distinguish sharply between 

 (a) mere surface invasions, (6) infection proper, and (c) infectious 

 disease. The first subject belongs essentially to the epidemiologist 

 and the sanitarian, while the study of infection and infectious disease 

 engage the attention of the immunologist. 



