THE PROBLEM OF VIRULENCE 7 



without infectious disease/' the distinction is probably one of degree 

 only there being some re action on the part of the host even in the 

 mildest cases, if only in the weakening by withdrawal of body sub- 

 stance, which distinguishes the infected from the uninfected animal. 

 In other cases there may even be advantage to the host, following the 

 infection, to the detriment of the invaduig micro-organism, a phe- 

 nomenon most clearly illustrated by the invasion of the root hairs of 

 leguminous plants by the Nitrogen-fixing " root-tubercle" bacilli, a 

 condition in which, as Fischer says, the plant may be regarded as 

 parasitic upon the bacteria. 



The actual harm resulting from the infection must, to a large 

 extent, depend upon the degree of adaptation to the new conditions 

 of life possible on the part both of the invader and of the host. If 

 the invader can acquire resistance to the defensive properties of the 

 host, and the latter can be similarly adapted to the harmful effects of 

 the invader, a prolonged condition of infection might ensue, a sort 

 of truce without manifestations of the disease. Although this is con- 

 ceivable, such mutual adaptation is probably very rare in human 

 disease. 



In cases of so-called chronic septicemia in which bacteria may 

 be again and again isolated by blood culture from the circulation it 

 is more than likely that the organisms are constantly present, not 

 because they multiply or maintain themselves within the circulation, 

 but rather because they are being continuously discharged into the 

 blood from an established focus in the tissues as, for instance, on a 

 heart valve. We have examined the serum of patients with subacute 

 and chronic septicemia (endocarditis), and often found powerful 

 opsonic action against the invading germs even when the patient's 

 own serum and leukocytes were used in the tests, evidence that the 

 bacteria were probably being successfully disposed of after they had 

 gained entrance into the blood stream. In rabbits, too, in our ex- 

 perience and in that of Miss Gilbert of this laboratory, it would 

 seem that protracted septicemia is present only when secondary foci 

 have been established from which the bacteria are constantly being 

 discharged into the blood. This we believe is rather the rule and 

 the establishment of a balance within the blood stream an exception. 

 When bacteria do succeed in withstanding successfully the opposing 

 forces active within the circulating blood their rapid accumulation, 

 the collapse of the defensive mechanism, and death of the patient are 

 probably the most common course. 



The point of view which we have expressed in the preceding 

 paragraph has been impressed upon us with particular insistence by 

 the observation of certain cases of bacteriemia following infections of 

 the middle ear, mastoid processes, and thromboses of adjacent veins. 

 In such cases it appears that the blood may be flooded with bacteria 

 which, nevertheless, disappear after the focus of infection has been 



