INFECTION. 41 



shown by the fact, established experimentally, that the 

 larger the number of bacteria introduced, the more speedily 

 does the death of the animal take place. 



With regard to the purity of the infecting material, the 

 question especially of mixed infection that is, infection with 

 a mixture of bacteria arises. In a number of diseases in 

 man several varieties of bacteria may almost always be 

 found in the disease-focus ; thus, for instance, in diphtheria 

 streptococci as well as diphtheria-bacilli, and in advanced 

 tuberculosis pyogenic cocci as well as tubercle-bacilli. The 

 presence of one variety of bacteria may facilitate the entrance 

 and the activity of others, whose virulence is increased by 

 the symbiosis ; in other words, infection with the one variety 

 of bacteria is rendered possible through the agency of the 

 other. Attenuated pyogenic cocci may be rendered again 

 virulent by the simultaneous introduction of bacterium coli, 

 of proteus, and even of saprophytes, such as prodigiosus, or 

 hay-bacillus. Streptococci are said to restore the toxicity 

 of attenuated diphtheria-bacilli when injected simultaneously 

 into guinea-pigs. The causative agents of typhoid fever 

 and of cholera regain their infectiousness when they are in- 

 troduced into animals in association with streptococci, coli 

 commune, or with metabolic products of proteus. Tetanus- 

 bacilli or tetanus-spores alone, without their toxins, do not 

 give rise to disease, as has been demonstrated experiment- 

 ally by Vaillard ; but if with them are injected other bac- 

 teria in themselves indifferent (as takes place in natural in- 

 fection through earth and splinters of wood), germination 

 takes place, with toxin-production and the development of 

 tetanus. For other anaerobic bacteria similar conditions 

 appear to prevail ; at least, it is possible to favor materially 

 infection with malignant edema and symptomatic anthrax 

 by simultaneous inoculation with pyogenic cocci, proteus, or 

 prodigiosus, or their metabolic products. With such mixed 

 infection, the clinical picture' the infection may be a 

 mixed one, as the result of the activity of the various bac- 

 teria. Thus, in the clinical picture of septic diphtheria the 

 distinctively septic symptoms are to be attributed to strepto- 

 cocci ; the intermittent fever of tuberculous patients, to pyo- 

 genic cocci. In other cases, however, the effect of the activ- 

 ity of the specific bacterium only may make itself manifest 

 in the clinical picture (as with tetanus), and the role of the 

 second is exhausted with the rendering possible of infection. 



