CONDITIONS MODIFYING INFECTION. 107 



and experimentally. It is evident, then, that as soon as the 

 natural resistance of the body is lessened infection is invited. 



d. Predisposition : In some instances there exists a predis- 

 position to certain diseases. Certain races and peoples have 

 an inborn susceptibility or resistance to some of the infec- 

 tious diseases. For instance, the colored races are immune 

 to yellow fever but very susceptible to smallpox. The fair- 

 skinned races, on the other hand, are extremely susceptible 

 to yellow fever. 



e. Heredity: It is believed by some that certain infectious 

 diseases (tuberculosis, syphilis) may be inherited by the off- 

 spring, from the father or mother, or from both. Consider- 

 able evidence has been offered in support of this supposition, 

 but when this evidence is weighed carefully it falls far short 

 of being conclusive. For the present such a theory is unten- 

 able. The child born of weak or sickly parents cannot possess 

 sufficient vitality to combat disease, and naturally will suc- 

 cumb to infection more readily than a child born of healthy 

 parents. With regard to syphilis, there is considerable evi- 

 dence at hand in support of the belief that it is . possible to 

 inherit the disease or to be born with active manifestations 

 of the disease. Space forbids a more detailed consideration 

 of this very interesting subject, and we must refer our readers 

 to the numerous dissertations on the subject which can be 

 found in the current literature. 



SOURCES OF INFECTION : As has been pointed out, the sur- 

 face of the body, the various mucous membranes, and all the 

 tracts which open on the surface of the body, harbor an 

 extensive and varied flora at all times. Nuttall has shown 

 that the newly born animal is absolutely free from bacteria, 

 but that before long it is the host of a large number of patho- 

 genic and non-pathogenic bacteria. Different parts of the 

 body harbor germs which produce diseases peculiar to that part, 

 and thus the individual may be the source of his own infection. 



Skin : On the skin we always find various species of staphy- 

 lococci, especially the Staphylococcus epidermidis albus. The 

 Bacillus graveolens is responsible for the odor of sweating 

 feet ; the Bacillus prodigiosus, of red sweat ; but neither of 

 these germs is pathogenic. It is very improbable, how- 



