December 31, 1920] 



SCIENCE 



631 



assisted by the natural defensive mechanisms 

 of the body, which also are roused into action 

 to overpower the invader and the cooperation 

 of which often insures protection against rein- 

 vasion, acquired at the end of, and in con- 

 sequence of, the struggle. 



INFECTION AND SURVIVAL 



Infection and the mastering of infection 

 are intricate biological processes in which 

 contending forces are brought into play one 

 against the other, whence a struggle ensues. 

 We have seen that the host stands ready 

 equipped with mechanisms of defense which 

 may be quickly mobilized, and which undergo 

 favorable modification during infection, when 

 as we say, it proceeds toward a favorable 

 termination. The bacteriologist has learned 

 within the past quarter of a century to 

 imitate nature's method of surmounting in- 

 fection by supplying certain of the defensive 

 implements artificially wrought to be brought 

 to her aid in time of stress and need, and the 

 chemist is learning more and more the man- 

 ner of adapting drugs to the destruction of 

 the microbic parasites of disease for a like 

 purpose. 



All the advantage is, however, not on the 

 side of the body, since the parasites also pos- 

 sess powers of modification, through which the 

 most elaborate obstacles placed in their way 

 by the host may be rendered futile. 



These adaptations consist in the acquisition 

 of special properties of aggressive action or 

 virulence, with which is associated the ability 

 to produce and liberate substances paralyzing 

 to the defensive processes of the host. Again, 

 the parasites may surround themselves with a 

 kind of mantle, protecting them from the po- 

 tentially destructive effects of serum and pha- 

 gocyte. Or they may undergo an internal 

 change of constitution, through which resist- 

 ance to injurious agencies not normal to the 

 species is developed. The last condition is 

 called " fastness " and has been observed espe- 

 cially among trypanosomes and spirochetes ex- 

 posed within the body of the host to ineffective 

 amounts of specific serums or chemicals. 



With so many factors interplaying, it is not 



difficult to perceive that the problem of in- 

 fection is a complex one, both as regards its 

 occurrence and its issue. But our under- 

 standing of the conditions under which it 

 arises has been immeasurably extended by the 

 discovery of the insect and higher animal 

 agencies in communicating infective agents 

 to man, and of the part played by so-called 

 microbe carriers, those unfortunate and in- 

 nocent persons who have recovered from or 

 merely been exposed to a communicable dis- 

 ease, or suffered a slight, abortive, or ambu- 

 lant attack of which they are ignorant, and 

 the discovery of the usual portals of entry into 

 the body of pathogenic microorganisms. 



Infectious diseases prevail in two more or 

 less distinct, but at times interwoven ways, 

 which we speak of as the sporadic and the 

 epidemic. The former represents the ordinary 

 manner of spread, the latter the occasional or 

 periodic explosive outbreak or wave, such as 

 has been experienced recently with the pan- 

 demics, of poliomyelitis, influenza and lethar- 

 gic encephalitis. 



I What has been sought in the past and is be- 

 ing assiduously looked for in the present is an 

 (adequate explanation of the transition from 

 ,the sporadic to the epidemic type of disease. 

 ,We possess already quite accurate numerical 

 .data which show the manner in which epi- 

 |demics begin, how they reach their maximum 

 ■or peak, and then how they fall away again. 

 (Indeed, we now construct easily and recognize 

 ^readily the epidemic curves of different epi- 

 .demic diseases. But it is to be hoped that a 

 .new era is appearing in the study of epidem- 

 iology in which experiment may play a part 

 along with observation, statistical and other. 

 Already beginnings are being made in the at- 

 tempt to define the distinction between the 

 potentially fluctuating grades or power of in- 

 fectivity and degree of virulence, taking the 

 former to mean the natural propensity which 

 p. microbe displays in penetrating the ordinary 

 portals leading into the body and its ability to 

 survive and multiply there, and the latter the 

 (Capacity to overcome the natural defenses 

 when artificially inoculated. This is a field 

 (dearly approachable by experiment, using 



