Pathogenesis 8i 



Plasmodia of malajria may cease any longer to appear in the 

 peripheral blood and the patient may cease to have any paroxysms 

 of the disease, yet a few of the parasites in the bone-marrow or 

 spleen continuing to multiply, may suffice to keep a few gametes 

 in the blood from which they may be taken by a mosquito, to be 

 passed after the necessary cycle of development in its body, to 

 other human beings subsequently bitten. 



Much of the future of sanitary science will have to do with the 

 discovery and proper treatment of " carriers " of the infectious agents, 

 from whom the public must be defended until the infectious agents 

 can be eradicated from their bodies. 



PATHOGENESIS 



This subject can be understood only through a broad knowledge 

 of the metabohc products of micro-organisms. In general it may 

 be said that the abihty of micro-organisms to do harm depends upon 

 the injurious nature of their products. This alone, however, will 

 not explain the phenomena of infection, for in many cases the in- 

 toxication is subsidiary in importance to the invasive power of the 

 •micro-organisms. Some bacteria having but limited toxic powers 

 possess extraordinary powers of invasion, as Bacillus anthracis, 

 and the intoxication becomes important only after the organisms 

 have penetrated to all the tissues of the body. Others, with more 

 active toxic properties, have but limited invasive powers, and a few 

 organisms, growing with difficulty in some insignificant focus, ex- 

 cite actively destructive reactions in the tissues with which they 

 come in contact. Still others, with limited invasive powers, elimi- 

 nate active toxic substances, soluble in nature, that enter the cir- 

 culation and act upon cells remote from the bacteria themselves, 

 as in diphtheria and tetanus. 



The invasive power of the organisms depends upon their ability 

 to overcome the body defenses. This may indicate activity of the 

 infecting organism, or weakness of the defensive mechanism. The 

 relation of these factors is exceedingly complex, only partly under- 

 stood, and wUl be fully discussed in the chapter upon Immunity. 



For convenience toxins may be described as intracellular or in- 

 soluble, and extracellular or soluble. 



The Intracelltilar Toxins. — Until the investigations of Vaughan, 

 Cooley and Gelston,* and later Vaughan and his associates, Det- 

 weiler,t Wheeler, J Leach,§ Marshall and Gelston, || Gelston,** J. 

 V. Vaughan,tt Wheeler,tt Leach,§§ Mclntyre,|i|| and others, it 



* "Journal of the American Medical Association," Feb. 23, 1901; "Trans. 

 Assoc. Amer. Phys.," 1901; "American Medicine," May, 1901. 

 t "Trans. Asso. Amer. Phys.," 1902. t IWd. 



§ Ibid. II Ibid. ** Ibid. 



tt Ibid. 



it "Jour. Amer. Med. Assoc," 1904, xlii, p. 1000. 

 §§ Ibid., p, 1003. 111! Ibid., p. 1073. 



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