CYCI.es of potency : PERIODS OF MALIGNANCY 

 AND BENIGNANCY. 



Nothing is more certain than the varying potency of a microbe 

 and its toxins for a year or a series of years at a time. For a 

 number of years just past smallpox has been so mild in the United 

 -States that physicians often differed in opinion as to whether the 

 prevailing affection was variola or a less dangerous skin disease. 

 Some years strangles are almost uniformly mild and simple and 

 in others almost constantly irregular and dangerous. The same 

 holds true of canine distemper, hog cholera, influenza, contagious 

 pneumonia, lung-plague, Rinderpest, and other pestilences. Even 

 locality and season influence largely. lyung-plague is a less vio- 

 ent affection in the Northern United States than in Great Brit- 

 ain, and Glanders largely escapes recognition in Wyoming and 

 Montana. Tuberculosis at one time, or in one locality, shows a 

 very low virulence and mortality, and at another time or place an 

 unwontedly dangerous and fatal prevalence. Again, the food, 

 or indoor life of an a,nimal, or other variation of management o 

 environment often modifles the susceptibility to a microbe and 

 therefore its virulence and the fatality it causes. Any such con- 

 dition prevailing for a length of time determines a special cycle 

 of the disease for evil or good, so that agencies which appear to 

 operate in the way of prevention or cure at one time are compar- 

 atively useless at others. This has often led to confident claims 

 for given methods, based on a series of experiments, or on obser- 

 vations made during a given epizootic which break down com- 

 pletely under a larger experience. 



This should never be forgotten in estimating the value of 

 measures which come short of the complete and final extinction 

 of the microbe of a disease. All restriction by special diet, med- 

 icine, open air ; all attempts at immunization by enfeebled mi- 

 crobes (misnamed vaccine), by sterilized toxin or cytase drawn 

 from such microbes ;- any use of antidotal agents in the form of 

 antiseptics, antitoxins, etc. ; any resort to sero-therapy or organo- 

 therapy will perhaps succeed in a long series of cases, and then 

 signally fail under new conditions. But when the one essential 



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