242 SUSCEPTIBILITY AND IMMUNITY. 



come by unusual virulence of the pathogenic infectious agent, or by 

 the introduction into the body of an excessive amount of a pure cul- 

 ture of the same. 



The pathogenic potency of known disease germs varies as widely 

 as does the susceptibility of individuals to their specific action. In 

 general it may be said that the more recently the germ comes from 

 a developed case of the disease to which it gives rise the more viru- 

 lent it is, and the longer it has been cultivated outside of the animal 

 body the more attenuated is its pathogenic power. Thus when the 

 discharges of a typhoid fever patient find their way directly to a 

 water-supply of limited amount a large proportion of those who 

 drink the water are likely to be attacked ; but when a considerable 

 interval of time has elapsed since the contamination occurred, 

 although the germs may still be present, the liability to attack is 

 much less on account of diminished pathogenic virulence. 



The development of an attack also depends, to some extent, upon 

 the number of germs introduced into a susceptible individual at one 

 time. The resources of nature may be sufficient to dispose of a few 

 bacilli, while a large number may overwhelm the resisting power of 

 the individual. 



The experiments of Cheyne (1886) show that in the case of very 

 pathogenic species a single bacillus, or at least a very small number, 

 introduced beneath the skin, may produce fatal infection in a very 

 susceptible animal, while greater numbers are required in those less 

 susceptible. Thus a guinea-pig succumbed to general infection after 

 being inoculated subcutaneously with anthrax blood diluted to such 

 an extent that, by estimation, only one bacillus was present in the 

 fluid injected ; and a similar result was obtained in mice with Bacillus 

 murisepticus. In the case of the microbe of fowl cholera (Bacillus 

 septicaemia hemorrhagica}) , Cheyne found that for rabbits the fatal 

 dose was 300,000 or more, that from 100,000 to 30,000 cause a local 

 abscess, and that less than 10,000 produce no appreciable effect. The 

 common saprophyte, Proteus vulgaris, was found to be pathogenic 

 for rabbits when injected into the dorsal muscles in sufficient num- 

 bers. But, according to the estimates made, 225,000,000 were re- 

 quired to cause death, while doses of from 9,000,000 to 112,000,000 

 produced a local abscess, and less than 9,000,000 gave an entirely 

 negative result. 



ACQUIRED IMMUNITY. 



It has long been known that, in a considerable number of infec- 

 tious diseases, a single attack, however mild, affords protection 

 against subsequent attacks of the same disease; that in some cases 

 this protection appears to be permanent, lasting during the life of the 



