August 5, 1921] 



SCIENCE 



103 



of transfer from one host to another, has been 

 greatly modified by civilization. It is there- 

 fore necessary in any attempt to formulate the 

 problems of etiology to take into consideration 

 the primitive conditions under which the in- 

 fectious diseases originally flourished and from 

 which they have come down to us. We do 

 not know whether they originated with man or 

 earlier among his progenitors. But it is safe 

 to take the ground that infectious diseases 

 flourished among the earliest races and that 

 they flourish to-day among savage and semi- 

 civilized peoples as they do among domesti- 

 cated animals in our midst. That is to say 

 the infectious agents developed in an environ- 

 ment in which transfer from host was direct, 

 immediate, and easily brought about. Fur- 

 thermore, the infusion of susceptible subjects, 

 except during wars and migrations, was slight. 

 These two conditions tended to counteract 

 one another and to bring to an approximate 

 perfection the parasitic habit of the invading 

 organism. 



It is this fourth stage of transfer that en- 

 gaged the entire attention of the early bac- 

 teriologists. They created the era of isolation 

 and disinfection by improving diagnostic meth- 

 ods and studying the modes of exit of infec- 

 tious agents and their resistance in transit. 

 It was tacitly hoped and expected in this 

 great work that infectious diseases could be 

 easily controlled and suppressed by destroy- 

 ing the agents in the environment of the sick. 

 The science of one generation becomes the 

 practise of the next. Disinfection, isolation 

 and the widening of the danger space between 

 the sick or infected and the well is the chief 

 occupation of modern sanitation. The actual 

 significance of this practise needs to be evalu- 

 ated from time to time if only in the interests 

 of economy of effort. While it is generally 

 conceded that the movement of the agents of 

 disease should be restrained as much as pos- 

 sible and while heroic efforts are being made 

 to this effect by health officials, economic forces 

 are driving people together and condensing 

 populations and thereby largely neutralizing 

 the efforts of sanitarians. If any value can be 

 put on this work at present, it might be to the 



effect that it tends to keep individuals from 

 getting an overdose of infection. 



When we come to the first phase in the 

 cycle, the entrance of the virus into the body 

 and its penetration through the skin and mu- 

 cous membranes, our knowledge is on the 

 whole neither accurate nor abundant. While 

 the fourth stage has been pretty thoroughly ex- 

 ploited, the first ia hardly at all known in its 

 details. Each well established infectious agent 

 will have its own story to tell of this phase. 

 In extenuation of the deficiencies of our exact 

 knowledge it should be stated that the problem 

 is a very difficult one. Microorganisms leave 

 the body in armies, having multiplied to sup- 

 ply a progeny ample to cover losses in transit. 

 On entering only single individuals or small 

 groups are involved and unless their mor- 

 phology is characteristic, like that of the 

 sporozoa and the metazoan parasites, the entry 

 is well beyond the ken of the observer. It can 

 only be got at indirectly. The difficulties of 

 this stage are well illustrated by the prolonged 

 discussions concerning the entry of tubercle 

 bacilli. Much has been done and written in 

 the attempt to clarify this problem. The doc- 

 trine of the inhalation of dried bacilli in 

 dust, the droplet infection of fresh sputum, 

 and the theory of the alimentary origin of in- 

 fection have had their day in court. Simi- 

 larly the portal of entry of the virus of the 

 eruptive diseases has been the subject of much 

 study and discussion. 



The penetration of living agents through 

 the skin and mucous membranes is full of un- 

 answered questions. The resistance of the 

 normal mucous coverings has probably been 

 greatly underestimated. The effect of injuries 

 in removing this barrier has been similarly 

 underestimated. From a practical standpoint 

 this problem may seem academic since the mu- 

 cous coverings for example may rarely be 

 free from minor lesions and such lesions may 

 remain, at best, undetected. But the object 

 of genuine medical science is to get away from 

 the benumbing influence of such considera- 

 tion. Do typhoid bacilli, for instance, pene- 

 trate the normal mucosa, or are they dependent 

 on slight lesions? Do protozoa assist them 



