IMMUNITY 263 



attenuated in most; generally, when existence in an unfavorable medium is of 

 short duration changes so effected are temporary, as manifested by a resumption 

 of primary virulence when restored to a favorable environment; prolonged 

 cultivation of bacteria in an unfavorable medium tends to produce permanent 

 changes in them which become hereditary. 



INFECTIOUSNESS OF BACTERIA 



If we employ the word infection to indicate not merely the lodgment of 

 bacteria upon or in tissue, but the subsequent multiplication of them with 

 injury to the host, important differences in the infectious power of different 

 species becomes apparent together with definite facts that determine in a given 

 case whether the lodgment of bacteria upon or in tissue can or cannot initiate 

 infection. 



Webb has shown that the injection of one, two or three tubercle bacilli 

 never produces tuberculosis; but the injection of 10 or more of the most virulent 

 tubercle bacilli regularly produces tuberculosis. If tubercle bacilli only half as 

 virulent are employed, however, the minimum number required to produce 

 infection is more than 20. Webb made these observations when experimenting 

 with monkeys. 



There is a great mass of evidence indicating that the probability of infection 

 following the lodgment of bacteria is in proportion to their number. It is 

 highly improbable that infection ever follows the lodgment of one or several 

 bacteria. 



Certain organisms manifest a predilection for certain tissues, thus a quantity 

 of rabies virus which will regularly produce disease when injected beneath the 

 dura, very rarely causes infection when injected subcutaneously or into a mus- 

 cle. Rapidly fatal, disseminated tuberculosis, as well as the chronic pulmonary 

 type, rarely shows involvement of muscles. 



The infectiousness of some bacteria is influenced by association or coincident 

 lodgment with other species of bacteria and medicinal agents. When inunctions 

 of mercury are applied at the point of entry within several hours after the lodg- 

 ment of the treponema pallidum infection is frequently prevented. If quinine 

 enters the body at the time tetanus spores gain lodgment, or shortly afterward, 

 the development of infection is favored. Though the influences causing it are 

 largely undeterminate, it is worthy of note in this connection, that the existence 

 of diphtheria seems to predispose to scarlet fever; that diphtheria, superimposed 

 on Vincent's angina, is unusually virulent; that the existence of gonorrhea favors 

 secondary infection of the urethra by the pseudodiphtheria bacillus. 



RESISTANCE OF MAN TO INFECTION 



Resistance to infection is both general and specific. Man naturally possesses 

 certain forces that tend to destroy any species of bacteria that gain lodgment 

 upon or in the body. Many bacteria are destroyed by the hydrochloric acid of 

 the stomach. Most pathogenic bacteria cannot cause disease until they have 



