106 INFECTION. 



abscess-formation. The same is true of every other patho- 

 genic organism. 



THE INDIVIDUAL : The susceptibility of the individual, 

 or his resistance to disease, materially modifies the occurrence 

 of infection. These conditions are dependent upon : 



a. Immunity : An individual may possess a natural or ac- 

 quired resistance to a certain disease, thus making the occur- 

 rence of infection with the organism causing that disease impos- 

 sible or, at least, difficult. The condition of immunity is not 

 absolute, as extremely virulent organisms will produce dis- 

 ease even in an immune. Usually, however, the disease then 

 occurs in a very modified form (see also Chapter XIII.). 



b. Vital condition: A healthy and vigorous person will 

 naturally resist invasion much better than one suffering from 

 some disease, such as cancer, diabetes, or a heart-, kidney-, or 

 liver-disease. Young individuals are more susceptible to 

 some diseases than older persons, and old people are attacked 

 by infectious disease which the younger individual escapes. 

 Women are more predisposed to some infections than men, 

 and some infections are seen much more frequently in men 

 than in women. Environment is largely responsible for this. 

 Women, being more confined to the house, naturally are 

 exposed to diseases which are the result of such confinement. 

 The lack of fresh air and exercise influences the vitality and 

 resistance of the woman. Diseases incident to exposure, 

 such as pneumonia, occur more often in men. Anything that 

 has a tendency to diminish bodily resistance in any way pre- 

 disposes to infection. 



c. Traumatism: This usually predisposes to infection by 

 creating conditions favorable to the development of bacteria. 

 The natural resistance of the uninjured tissue to infection is 

 diminished by the traumatism, and also the shock incident 

 thereto. The unbroken skin never serves as an infection 

 atrium, whereas infection is invited by an injury. These inju- 

 ries include operations. Tuberculosis of the bones and 

 joints often follows an injury of some kind even when the 

 skin is not broken. Malignant, ulcerative endocarditis never 

 results unless there has been some previous injury of the 

 heart valves. This has been demonstrated both clinically 



