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 wsimilar result was obtained in mice with Bacillus 
murisepticus. In the case of the microbe of fowl cholera (Bacillus 
‘septiceemia hemorrhagice), 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 Jess 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 protéction appears to be permanent, lasting during the life of the 
