108 SAPROPHYTISM, PARASITISM, AND PATHOGENISM 



frequently attributable to unusually direct transfer of organisms by 

 a common vehicle through a series of susceptible hosts than to the 

 escape of the microbes from one host to another. Thus, milk-borne 

 epidemics of septic sore throat may be extensive and involve many 

 patients, but secondary transfer from man to man is relatively uncom- 

 mon. These bacteria have not, as a rule, perfected their mechanism 

 of escape from the tissues of one host to those of another. The 

 epidemics are usually of brief duration and it is probable that the 

 surviving microbes return to their original parasitic state. 



Of far greater importance is a probable tendency of many progres- 

 sively pathogenic bacteria to act more and more on the defensive; 

 to gradually disembarrass themselves, on the one hand, of the offen- 

 sive weapons which originally conferred upon their possessors the 

 ability to invade their host, and, on the other hand, to perfect what- 

 ever defensive weapons they may have possessed the rudiments of. 1 

 Such a change, as Theobald Smith has pointed out, would be difficult 

 to detect, because an elimination of the more aggressive type and its 

 gradual replacement by a strain in which the defensive elements were 

 more prominently represented would require years for its accomplish- 

 ment. Such a change in the activities of the microorganisms would 

 probably be accompanied by reciprocal activities of the host, so that 

 eventually a strain of microorganisms would be evolved which had 

 reached a state of relative equilibrium with the host. Unusually 

 virulent strains of microbes would tend to perish with their hosts, 

 and unusually susceptible hosts would tend to perish with their 

 invaders. A mutual adjustment of virulence and resistance between 

 the surviving hosts and microbes would lead eventually to one of 

 three conditions : 



1. Gradual extinction of the microorganism; 



2. The gradual assumption of a parasitic or "opportunist" exist- 

 ence, or 



3. A more perfect pathogenism in which the mechanism of invasion, 

 multiplication within the tissues and escape to other hosts is accom- 

 plished without acute damage to the host. 



It might well happen that the introduction of such "balanced" 

 strains into new fields would lead to temporary disaster, as for 

 example, the highly fatal epidemic of measles when this virus first 

 gained a foothold in the South Sea Islands. 



1 Theobald Smith (Some Problems in the Life History of Pathogenic Microorganisms, 

 Am. Med., 1904, viii, 711) clearly stated and discussed this hypothesis over a decade 

 ago, and it is surprising how little cognizance has been taken of it. 



