102 



SCIENCE 



[N. S. Vol. LIV. No. 1388 



animals only a certain per cent, contract the 

 disease and only under certain conditions in- 

 evitably leads to an inquiry into the causes 

 other than the mere presence of infectious 

 agents. 



At this point it might be well to call atten- 

 tion to the necessity for including all invasive 

 living agents however diverse in a study of the 

 factors leading to disease and recovery. In 

 pathology it has been customary to distinguish 

 between parasitic invasion and bacterial infec- 

 tion, the former producing relatively slight 

 disturbances in the host, the latter the acute, 

 often highly fatal epidemic and epizootic dis- 

 eases. The distinction is useful but it can 

 be made to apply only to extremes. Grada- 

 tions of all shades occur. For the sake of a 

 more exact terminology, the distinction be- 

 tween invasion and infection might be made to 

 hinge upon the capacity of parasites to mul- 

 tiply in the host. Thus the number of nema- 

 todes and related worms in the host is not 

 larger than the number of fertile eggs intro- 

 duced or of those individuals which actively 

 penetrate as larvae.^ The formidable power of 

 the infectious agents is due to their capacity 

 for indefinite multiplication in the host. Sex- 

 ual reproductive stages are not known. Cer- 

 tain protozoa such as sporozoa not only pass 

 through sexual stages, but they may also mul- 

 tiply asexually and more or less indefinitely 

 in the host tissues. They may be considered 

 both invasive and infective. It is thus best 

 to class all living invasive organisms as para- 

 sites, subject more or less to the same host 

 mechanisms of repression and destruction. At 

 one end of the scale are the highly specialized 

 forms, adapted to one host species or even 

 one race. At the other end are types emerg- 

 ing from the predatory or saprophytic stage 

 and acquiring parasitic habits. 



Another concept which we as medical men 

 should have clearly before us is that the phe- 

 nomena which medical science is chiefly in- 

 terested in, namely, those of disease, are merely 



- We must except from this broad statement the 

 nematodes of the genera filaria and trichinella 

 since their progeny develops within the same host 

 to the larval stage and stops there. 



epiphenomena in an evolving parasitism, by- 

 products which tend to lessen and disappear 

 as the parasitism approaches a biological bal- 

 ance or equilibrium. How rapidly this evo- 

 lution may progress we have no means of 

 knowing. We do know that among animals 

 epizootics tend towards a lower level of mor- 

 tality and morbidity. If we are actually 

 studying by-products, it is obvious that to 

 understand them we must first understand 

 the main processes that give rise to the by- 

 products and that disease is studied most 

 successfully by studying the necessary con- 

 ditions that give rise to it. If the by-products 

 are in themselves necessary antecedents of 

 other pathological conditions, they would of 

 necessity be included in any study of causa- 

 tion. 



A sufScient number of living agents of dis- 

 ease and of parasites has now been studied to 

 permit a tentative classification into highly 

 specialized parasites adapted to and dependent 

 on a given host and those that are more or less 

 predatory, awaiting adaptation provided their 

 organization should permit it. The highly 

 adapted microorganism which depends upon 

 one host for its existence, as, for example, 

 the still unknown smallpox organism, has 

 through natural selection established between 

 itself and its host a certain balance or equi- 

 librium. This can be deiined as a condition 

 of both host and parasite which permits the 

 latter to enter the body, multiply enough and 

 escape so that the arrival of its progeny in 

 another host is assured. On the basis of this 

 relationship we may define four critical phases 

 in the life cycle of the microbe: first, its 

 entry into the body and through protecting 

 tissues; second, its transportation to and mul- 

 tiplication in certain tissues; third, its escape 

 from those tissues and from the host as a 

 whole ; and fourth, its transfer to another host. 

 Each one of these phases is capable of sub- 

 division into a larger or smaller number of 

 sequences according to the special living agent 

 involved. In the insect-borne diseases the 

 insect acts as transfer agent and as introducer 

 into the blood. The fourth and the first stage 

 merge. In general the fourth stage, or stage 



