156 MANUAL OF BACTERIOLOGY. 



than adults, though apparently nearly exempt from the exan- 

 thematous fevers during' the early weeks of life. Osteomyelitis 

 is commoner in infants than in adults, as also is tuberculous 

 meningitis. 



How much influence is to be ascribed to individual pre- 

 disposition in contracting or warding off infection is uncertain. 

 Welch says: "The fact that some individuals are attacked, and 

 others, apparently equally exposed to the danger of infection, 

 escape, is not always due to any especial predisposition on the 

 part of the former. It may be that the germs hit the one and 

 miss the other, and we would have no more right to say that the 

 former are especially predisposed than to say that those who 

 fall in battle are predisposed to bullets and those who escape 

 are bullet-proof." It is probable that the importance of an 

 hereditary tendency to certain infections, notably tuberculosis, 

 has been overrated. 



Race. — The influence of racial predisposition is undeniable. 

 For example, it is known that the negro race is much less sus- 

 ceptible to yellow fever than the white race. 



Local conditions often have a most important influence 

 in determining the occurrence of infections. In endocar- 

 ditis the lesion usually occurs along the line of closure of 

 the heart-valves, indicating that the point subjected to the 

 greatest friction is the part of the endocardium most liable 

 to infection. Regions where there is passive hyperemia are 

 more vulnerable, as is seen in hypostatic pneumonia. Lo- 

 calities which have suffered from previous inflammation or 

 irritation are rendered more liable to subsequent infection, 

 as when the bladder or pelvis of the kidney containing a calculus 

 becomes the seat of a suppurative cystitis or pyelitis. 



Local conditions become of great importance in surgery. 

 The surgeon can seldom be certain of dealing with a perfectly 

 aseptic wound, and must rely to a large extent upon the power 

 inherent in the fluids and tissues to prevent the development 



