in any region of the body ; they correspond with those of the interstitium 

 of the lungs. Recovery from these lesions are succeeded by immunity, 

 that is to say, a subsequent inoculation within a reasonable short time 

 no longer produces the typical lesions, neither does an animal so treated 

 contract lung-sickness within that time. When at a later period, a year 

 or more, the same animal is again injected, a reaction can take place, 

 or when exposed to natural infection, it may contract pleuro-pneumonia. 

 We thus stand here before the fact that an acquired immunity 

 diminishes in the course of time. This, of course, varies with the animal, 

 and in some occurs sooner than in^others. Immunity acquired by recovery 

 from the disease naturally acquired lasts, according to Nocard and 

 Leclainche,* three to four years. 



Immunity to Different Quantities of Virus. 



Quarter-evil in cattle is due to the introduction of a specific bacterium 

 into the subcutaneous tissue of young cattle. Recovery from the lesions 

 thus produced is succeeded by immunity. This immunity can be brought 

 about in various ways ; one of them is the introduction of the virus in 

 small quantities, and here the observation is made that the immunity 

 stands in direct relation to the quantity of virus injected. Thus, if the 

 dose of virus in increased, the immunity previously obtained does not 

 protect, and breakdowns occur (Kitt). This fact is generally observed in 

 diseases caused by bacteria ; an immunity obtained through the introduc- 

 tion of the minimum lethal doses will protect against this dose, or perhaps 

 a small multiple of it, but not so against a larger one. 



Immunity to Different Virulency. 



Another contingency which occurs in experiments with bacteria is 

 the different virulency of one and the same bacterium. Artificial 

 immunity against anthrax, for instance, is obtained by a double 

 injection of a first and second vaccine, differing in virulency. This 

 differentiation in virulency can be so exalted that the immunity thus 

 obtained can be broken by such a virus. This exaltation can be obtained 

 by passing the virus through a series of susceptible animals of the same 

 or jof different species. It is noted and generally accepted that the more 

 virulent the virus, the more sohd becomes the resulting immunity. 



The Immune Animal as a Reservoir of Virus. 



An important fact to be considered in connection with the immunity 

 obtained, either naturally or by inoculation in the above-mentioned 

 diseases, with the exception of pleuro-pneumonia, is the observation that 

 these animals no longer propagate the disease. The principle holds also 



* Nocard and Leclainche, " Les Maladies Microbiennes les Animaux,'! .1903. 



