PHENOMENA OF INFECTION. 91 



ment dissimilar in their action. Previous disease, 

 alcoholism, injuries, and operations, all portend the 

 same end, viz., a lessening of the natural resistance. 

 The blood and its cells contain the largest amounts 

 of those active principles which combat disease, and 

 when it wastes, as it does in every disorder, this func- 

 tion is depressed or suspended. Besides being 

 ''thinned" by disease, the blood suffers impoverishment 

 through poisons (e.g., alcohol, lead); through the 

 absence of either sufficient or nourishing food; through 

 unhygienic surroundings, and by actual loss in volume 

 and cellular constituents in haemorrhages consequent 

 upon operations and injuries, and also blood sucking 

 parasites. In unhygienic surroundings perhaps the 

 most potent evil is the absence of pure air (oxygen) in 

 sufficient amounts to meet the normal needs of the body. 

 Typhoid fever furnishes a remarkable example of the 

 predisposing influence of a disease. After an attack 

 of typhoid, a person is particularly prone to diseases 

 of the biliary passages, such as inflammation of the 

 gall-bladder (cholecystitis) and gall-stone formation 

 (cholelithiasis). Predisposition is brought about in 

 the following way: During the fever, typhoid bacilli 

 find their way into the gall-bladder, where they are 

 liable to remain for months and even years, and besides 

 constituting a nidus there for the formation of gall- 

 stones, may also at any time give rise to either a catar- 

 rhal or suppurative inflammation of this organ. 



