INFECTION. 37 



infectious tetanus-material may gain entrance into a mem- 

 ber, there remain without effect, and when, later, a con- 

 tusion or some other injury of the member takes place, 

 tetanus may long afterward occur. A similar latent period is 

 probably also responsible for the observation recently made 

 of the presence of tubercle-bacilli in the lymph-glands of 

 apparently healthy individuals. Thus, the entrance of bac- 

 teria into the body does not invariably give rise to infec- 

 tion ; but, on the contrary, the conjunction of a whole series 

 of circumstances is necessary in order that infection may 

 take place. Of such circumstances, which are related partly 

 to the infecting material and partly to the infected indi- 

 vidual, the following are thus far known : 



(a) The Virulence of the Infectious Agent. The viru- 

 lence ol bacteria is a varying one. The degree of toxicity 

 possessed by a bacterial culture obtained from the diseased 

 focus decreases progressively : in the case of some bacteria 

 more rapidly (as, for instance, diphtheria-bacilli, and most 

 rapidly pneumococci) ; in the case of others, more slowly 

 (an anthrax-culture, for instance, will remain toxic for 

 weeks, a tetanus-culture for many months ; a culture of 

 tubercle-bacilli, with suitable transplantation, will still be 

 capable of infection after the lapse of years). The reduc- 

 tion in virulence is frequently accompanied by a corre- 

 sponding reduction in activity of growth, though not 

 always. Diphtheria-bacilli, for instance, are said to grow 

 more luxuriantly upon artificial culture-media the more 

 their virulence is diminished. The variation in degree of 

 the virulence of bacteria can, therefore, not be dependent 

 upon their varying activity of growth. The virulence must 

 rather be considered to correspond with the capability of 

 toxin-production : the more virulent bacterium generates a 

 more active poison or a greater amount thereof than the 

 less virulent organism. The reduction in virulence in arti- 

 ficial culture is prevented in the case of many bacteria by 

 frequent transplantation upon fresh nutritive media ; it may, 

 therefore, be in some way related to exhaustion of the 

 culture-medium, a want of appropriate nutritive material, 

 an accumulation of inhibiting metabolic products. Such 

 an interpretation is applicable also to the marked reduction 

 in virulence that is often observed in culture-media con- 

 taining grape-sugar ; the process of fermentation appearing 

 to influence the media in a manner unfavorable for toxin- 



