1915] on Anti-Typhoid Inoculation 425 



iu the blood, tetanus bacilli conversely are harmless in the intestine, 

 but most dangerous in the tissues. 



(c) The virulence of the invading organism. The importance of 

 this factor is illustrated bv hydrophobia, from the developed disease 

 of which no case of recovery has ever been recorded. 



(d) Physiological resistance of the invaded person — a subject to 

 which we shall revert in a moment. 



Varieties of Immunity, 



There are two main varieties of immunity — the natural and the 

 acquired. 



{a) Katural immunity, again, may be racial or physiological. 



1. The racial is illustrated by the Barbary sheep in its insus- 

 ceptibility to anthrax, which is so fatal to other species of sheep. 

 Another striking difference is that between such closely allied species 

 as the field mouse and the house mouse : one succumbs rapidly to 

 septicemia after a coccal injection to which the other is relatively 

 indifferent. 



2. Physiological resistance plays a most important part in the 

 difference in susceptibility of individuals of the same species. This 

 difference is most strikingly demonstrated by an instance reported 

 from India, where cholera dejecta carelessly thrown from a tent 

 contaminated a camp water-supply. Of the twenty-three persons 

 who drank from this supply on the following morning, only eight 

 developed cholera. 



Further illustration is afforded by the familiar occurrence of boils 

 in the debilitated and the diabetic. 



{h) Acquired immunity may be obtained naturally or artificially. 



1. If naturally acquired, it follows, and results from, an attack of 

 acute specific disease. In some instances — as, e.g., influenza — the 

 immunity is so short that an attack has even been supposed to leave 

 increased susceptibility to fresh invasions by the same organism. It 

 is generally recognized, however, that in the great majority of acute 

 infective diseases one attack confers a high degree of protection 

 against a second attack of that disease. In very rare cases a second 

 attack has been known to occur even in diseases conferring so 

 marked a subsequent immunity as scarlet fever or enteric. As such 

 exceptions but prove the rule and afford no proof that innnunity 

 is not thus conferred in general, so the rare instances of enteric 

 occurring in inoculated men cannot be regarded as arguments proving 

 that inoculation is valueless. It would be unreasonable to demand 

 more protection from artificial methods than is given by the natural 

 processes which they aim at copying. 



2. Artificially conferred immunity may be of the passive or the 

 active variety. 



The former is a curative and temporary result of the injection 



