112 APPLIED BACTERIOLOGY 



throughout. What the temperature should be is still a 

 matter of discussion. Many common disorders, such as 

 typhoid, diphtheria, and cholera, are with certainty dis- 

 infected by almost momentary exposure to temperatures 

 below 100° C. This is not the case with all ; dried tubercu- 

 lous matter, for instance, having been known to resist over 

 three hours' boiling, and our knowledge of the organisms 

 producing many diseases, for example small-pox and scarlet 

 fever, is at present insufficient to justify a definite state- 

 ment of the temperatures necessary for their disinfection. 

 The latest researches (Miquel and Lathrage) conclude that 

 twenty minutes' exposure to a temperature of 110° C. should 

 be allowed in all cases. A description of the best known 

 forms of steam disinfectors will be found in the Appendix. 



Immimity and Susceptibility. — In considering the action 

 of disease germs on animals, one cannot fail to be struck 

 with the remarkable differences which the same organism 

 produces when injected into different animals. 



The organism that invariably produces a fatal disease in 

 one animal will, when introduced into another animal, 

 either produce a mere local affection of no particular 

 moment, or possibly no effect whatever. 



For example, a virulent culture of the Bacillus tubercu- 

 losis, if inoculated into a guinea-pig, will produce general 

 tuberculosis, resulting in the death of the animal ; but in 

 man, a local tubercular infection, as, for instance, a post- 

 mortem wound, usually produces only a slight local lesion, 

 which after a time heals up completely. 



When an organism is capable of producing specific 

 disease in an animal, that animal is said to be ' susceptible ' 

 to that disease. 



The ' susceptibility ' varies greatly in degree, even for the 

 same kind of animal, the following being some of the most 

 important factors regulating the degree of susceptibility : 



