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SCIENCE 



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



at certain seasons? Do phagocytes ever mi- 

 grate out under normal conditions and carry 

 bacilli back into the tissues? Might this take 

 place when inflammatory processes are active ? 

 Is the entry of certain viruses prepared by 

 other viruses acting only on the epithelium 

 and destroying it or in some other way making 

 the tissues involved more vulnerable? Epi- 

 thelium-destroying parasites are well known 

 among the sporozoa. It is not improbable that 

 other types of microorganisms, especially 

 those not within the range of the microscope, 

 are specifically adapted as genuine parasites 

 to invade these cells and so prepare the way 

 for the saprophytic, predatory types which 

 are readily recognized because easily cultured 

 and therefore regarded as summing up the 

 entire etiology. 



After the living agencies have entered the 

 tissues they must run the gauntlet of the 

 blood, lymph, and the phagocytic cellular ele- 

 ments to reach those tissues where they mul- 

 tiply. Multiplication is essential, for a large 

 progeny is necessary to cover the losses in 

 transit. This stage of multiplication involves 

 the problem of specific resistance or immunity 

 and susceptibility, and also the practical prob- 

 lem of treatment by therapeutic agents, ser- 

 ums, vaccines, and the like. The biological 

 requisite to be fulfilled by the parasite in this 

 second stage, or stage of multiplication and 

 sexual development within the host, if such a 

 stage exists, is that multiplication must take 

 place in such a way that escape in large num- 

 bers from the host after the parasites have as- 

 sumed a more or less resistant form becomes 

 possible. This is best accomplished when they 

 settle down and multiply near some portal of 

 exit, first the skin or subcutis, second the respi- 

 ratory tract, third the digestive, and fourth 

 the genital tract. A brief consideration of 

 the localization of the different groups of 

 parasitic agents will show that these various 

 superficially located tissues are the chief seats 

 of multiplication. Localization in other tissues 

 or organs is rare and so far as the parasite is 

 concerned abnormal. If it should happen that 

 a race of Treponema pallidum arose which 

 promptly and exclusively localized in the cen- 



tral nervous system, it would die out for want 

 of an exit to another host. The tendency to 

 locate and multiply in tissues of lower vital 

 dignity, i.e., near the surface of the body or the 

 mucous membranes, safeguards the host as 

 well as the parasite. 



Among the parasitic invaders of man and 

 the higher animals the metazoan and protozoan 

 parasites have developed relatively perfect, in 

 some cases complicated cycles. The same is 

 probably true of those highly specialized micro- 

 organisms which produce the eruptive diseases 

 and of some of the so-called filterable viruses. 

 In fact, all diseases or parasitic states which 

 maintain themselves indefinitely in a host spe- 

 cies and are manifestly transmitted from case 

 to case have complete, even though not neces- 

 sarily elaborate cycles. 



The tubercle bacillus is frequently referred 

 to as a highly adapted parasite, but its para- 

 sitism is crude compared with that of the 

 smallpox organism. ' It has no well-defined 

 cycle except in phthisis, in which it is inhala- 

 tion and expectoration. If for the sake of 

 illustration we conceive the primary lesion as 

 leading in every case to a secondary miliary 

 tuberculosis in which there is extensive in- 

 vasion of the skin with subsequent ulceration, 

 the tubercle bacillus would then be inhaled 

 and after multiplication shed from the skin, 

 in so far approaching the smallpox organism 

 in its cycle. But there is no indication that 

 such a complete cycle ever will be established. 

 On the other hand, the leprosy bacillus ap- 

 pears to have in a bungling way reached this 

 stage, for the shedding of leprosy bacilli takes 

 place from skin and mucous membranes, no- 

 tably, of the nasal passages. 



In certain cases the cycle is limited to a 

 mucous membrane parasitism and the disease 

 is the result of a diversion of the organism 

 into the tissues of the body. The cycle of the 

 typhoid bacillus may be some locus of the 

 digestive tract with incursions into the blood. 

 Whether the invaders perpetuate themselves, 

 i.e., escape again, depends upon ulceration of 

 the lymphoid tissue of the intestines. The 

 meningococcus resides in the upper respira- 

 tory tract. We do not know whether the cocci 



