THE CAUSATION OF DISEASE. 171 



porary or permanent. It is well known that an individual will 

 sometimes escape unhurt when exposed to a given mal-E, while 

 on other occasions this same E will cause disease. These differ- 

 ences are due to temporary modifications of S. At one time an in- 

 dividual may be S, as regards a given mab-E, at another S 1+3 , so 

 thatE. , which, in the one instance, is powerless to harm him, will, 

 in the other, cause disease, owing to temporary structural altera- 

 tion. But, in such cases as these, the effective mal-E, although 

 apparently the same, is not actually so ; for the temporary altera- 

 tion in S must itself be due to temporary peculiarity of E, and 

 this latter may be unwittingly excluded from the sphere of causa- 

 tion ; yet, if it predisposes to the disease, it ought obviously to be 

 included under the specific pathogenic E. A child, for instance, is 

 on two separate occasions equally exposed to the scarlatina E. 

 The first time it escapes unhurt, but on the second occasion con- 

 tracts the disorder. There must, therefore, be some difference 

 in S on the two occasions. On examination it is perhaps dis- 

 covered that in the one instance the child was exposed to the 

 influence of the poison on a full stomach, but that on the second 

 occasion it had been without food for some time. Now it is ob- 

 vious that this want of food ought to be included with the specific 

 virus as part of the pathogenic E, since in this case the two 

 together were necessary to produce the disease. To give a 

 further illustration of this point. Troops under privation and 

 exhaustion are more liable to suffer from dysentery than others, 

 and, therefore, the privation and exhausting influences, as well as 

 the virus, must be included under the effective mal-E. Neverthe- 

 less in such cases we probably have actually to do with temporary 

 structural alterations induced by E. 



It may be asked what place in the above list external vio- 

 lence ought to occupy as a mal-environment ? Traumatism 

 is so potent to produce disease that we shall perhaps be inclined 

 to give it a high numerical value. The fact is, however, that 

 the numerical value of S is equally high, for the S is very 

 susceptible to external violence, and this susceptibility varies 

 with the individual. A twist that would snap the limb of an 

 infant would have no effect upon that of an adult ; a bullet that 

 would flatten impotently against the side of a hippopotamus would 

 destroy most other animals. But there is no occasion whatever 



