August 5, 1921] 



SCIENCE 



105 



which enter the central nervous system escape 

 or not. This is an important question for it 

 is probable that the degree of virulence of the 

 microbe depends upon some contact with the 

 host tissues more intimate than that of a mere 

 saprophytic existence in recesses of the mu- 

 cous membranes. If such an organism could 

 associate itself with some living or dead fac- 

 tor which assisted its entrance and exit, a 

 definite disease might thereby become estab- 

 lished, and we might expect the disease to rise 

 and fall in epidemic style according as the 

 helping factor is active or not. Contact with 

 and multiplication in living tissues, either 

 cellular or humoral, appears furthermore to 

 stabilize the microorganism. If we look over 

 the immunological and serological data which 

 have accumulated about the various groups 

 of parasitic agents we find the relationships 

 established by these reactions much more uni- 

 form among strictly invasive than among the 

 secondary organisms which depend upon for- 

 midable factors to open the way into the sys- 

 tem for them. Or to put it another way, 

 there is a much larger number of serological 

 types among the hangers-on than among those 

 capable of direct invasion. 



If the picture I have drawn of the para- 

 sitic cycle is fairly accurate, it follows that 

 the localization of disease agents or parasites 

 in all tissues and organs except those from 

 which ready escape to the exterior is pos- 

 sible is abnormal and unnecessary for the con- 

 tinued existence of the parasite. Hence, typi- 

 cal, characteristic, recurring infectious or 

 parasitic diseases affecting the central nervous 

 system, the ductless glands, the liver and kid- 

 neys, the muscular system and the joints do 

 not occur excepting as secondary localizations 

 of diseases involving the more external tissues. 

 If such localization should occur regularly 

 without some superficial localization as well, 

 we must look for some source of infection be- 

 longing to another species in which the cycle 

 is normal and from which a constant supply 

 of parasites is available even though they fail 

 to escape from the new host. The invasion 

 of the human body by the bovine tubercle ba- 

 cillus is a familiar case in point. In some 



infectious diseases several invasions of the 

 same host may be necessary to bring the para- 

 sitic cycle to completion or, viewed from a 

 medical standpoint, to bring the disease to the 

 clinical level. In bovine tuberculosis, which 

 can be studied anatomically and topograph- 

 ically by killing animals in early stages, the 

 first tubercle bacilli to enter the system usu- 

 ally land directly in regional lymph nodes. 

 To complete the cycle the virus must enter 

 the blood and establish secondary foci in lunga 

 or other tissues from which the bacilli may es^ 

 cape to another host. The cycle is, however,, 

 more easily explained by reinfection. The in- 

 haled bacilli lodge now in the lung tissue due 

 to a changed reaction of the host tissues at 

 the point of entry and the cycle — inhalation 

 and expectoration, assisted by discharge in the 

 feces through swallowing the sputum — is es- 

 tablished. 



There are two factors that may modify the 

 normal cycles more or less. One is a relative 

 immunity, which may be either natural or ac- 

 quired. Through immunity cycles may be 

 cut short chiefly in the stage of multiplication 

 and the parasite fail to escape at all or in 

 sufficient numbers to maintain further exis- 

 tence. If for any reason the normal resistance 

 of the host is reduced, the parasite may multi- 

 ply unduly or invade more territory and cause 

 death of the host before the cycle is com- 

 pleted. This condition may be explained by 

 regarding parasitism not as a condition of 

 peace but of armed truce. As soon as one 

 of the two organisms falls below a certain 

 level, the other takes the advantage. In case 

 the microorganism gets the advantage, it may 

 be fatal for both host and parasite. This 

 latter condition of reduced natural resistance 

 is supplied by civilization either by bringing 

 into the original disease other parasites and 

 greatly complicating what might have been a 

 simple situation, or else by conditions arising 

 from inherited defects, over-exertion, abnormal 

 diet, exposures to heat, cold, which are sup- 

 posed to favor the parasite in its entry and 

 multiplication. These conditions furnish the 

 many modifications of disease types which may 

 be as varied as the number of individuals at- 



