INFECTION AND INFECTIOUS DISEASE 21 



disease. Then, in the normal intestinal contents there are myriads 

 of bacteria, the majority of them harmless saprophytes, it is true, 

 but in addition there are 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 

 organisms 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 w r hen infection has once taken place then we may also find 

 clinical evidence of such infection, i. e., symptoms of the corre- 

 sponding infectious disease; but it does not follow because infection 

 has taken place that symptoms of disease must of necessity 

 develop. 



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 in sheep, for example, the infection may nevertheless result 

 fatally, but the period of time during which the animal shows clinical 

 symptoms of infection is so brief that one can hardly speak of evi- 

 dence 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. The same may at times be noted in 

 partially immunized animals, when extensive infection of the peri- 

 toneal cavity may be produced in the case of such organisms, as the 



