INFECTION AND DISINFECTION. 773 



acted like parasites, and exhausted the system during their develop- 

 ment. It is now, however, more commonly believed that the organ- 

 isms elaborate a special ferment or poison, which, when produced in 

 sufficient amount, gives rise to the symptoms of the disease. 



One of the most valuable results of the study of these organisms is 

 the discovery that by cultivation the virulence of some, at least, can 

 be so mitigated that when inoculated they produce only slight and 

 non-fatal symptoms, the development of which in a given animal is 

 nevertheless protective against future attacks of the original disease. 

 By cultivating the organisms of splenic fever at a temperature of 108°, 

 it is found that filaments are produced but not spores, and that by re- 

 peated cultivation this growth becomes altered as regards its properties 

 of causing disease. When inoculated it sets up a mild form of splenic 

 fever, not dangerous to life, but perfectly protective against subsequent 

 inoculation with the otherwise poisonous organisms. This discovery is 

 worthy of being classed with that of vaccination as a protection against 

 small-pox. 



With regard to the channels through which the contagious organ- 

 isms are spread, a few words will suffice to state what is known on 

 this point, which is intimately connected with the subject of disinfec- 

 tion. Air and water are the chief media for the propagation of infec- 

 tious disease. In the case of scarlet fever, which has been taken as 

 the type, the scales detached from the skin and similar tissues from 

 the throat contain the germs of the disease, and these find their way 

 into the atmosphere and are received into the lungs. They attach 

 themselves also to articles of clothing and furniture, and are thus 

 often carried long distances. In the cases of cholera and typhoid 

 fever, the discharges from the patient find their way into water, 

 which thus becomes the channel by which the diseases are propa- 

 gated. Food, too, may become similarly contaminated. Milk, for 

 instance, has been often known to convey the poisons of typhoid 

 fever, of scarlet fever, and of diphtheria. In the case of the first, the 

 contamination has been probably due to adulterating the milk with 

 foul water containing the disease-germs, but it may have arisen in 

 some cases from the typhoid emanations having been absorbed by the 

 milk. The poisons of scarlet fever and diphtheria were probably 

 transmitted to the milk from the skins and throats of persons em- 

 ployed in the dairy and recently convalescent or scarcely recovered 

 from attacks of these diseases. The germs of certain other infectious 

 diseases find their way into the system through abraded surfaces of 

 the body. 



The fatal character of many infectious diseases, and the ease and 

 rapidity with which they spread and attack large masses of the popu- 

 lation, are sufficient to account for the endeavors that have been made 

 since very early times to arrest their progress. As in many other mat- 

 ters, practice has preceded science, and, centuries before the vaguest 



