December 16, 1904.] 



SCIENCE. 



827 



■of entry and exit are circumvented and 

 the bacteria injected into the body and 

 withdrawn therefrom directly. As a result 

 ■of such passages many species of bacteria 

 have been made more virulent, and Pasteur 

 was able to greatly modify the unknown 

 virus of rabies. 



Besides the maintenance of virulence and 

 its occasional augmentation, a slow decline 

 to complete loss of virulence may be looked 

 for under conditions abnormal for the mi- 

 crobe. This probably goes on where the 

 bacteria multiply, partly or wholly pro- 

 tected fi'om bactericidal influences. The 

 bacilli of tuberculosis, which multiply in 

 •cavities of the' lungs or in muco-pus of the 

 air-tubes in chronic cases, must be regarded 

 as degenerating in virulence. And we 

 .actually encounter races varying consider- 

 ably in pathogenic power. In the throats 

 of well persons or those who had diph- 

 theria months ago, bacilli without any 

 power of toxin production, but with all 

 the other characters of genuine diphtheria 

 bacilli, are occasionally encountered. 



During the elimination of the more viru- 

 lent races of microorganisms, there goes on 

 as well a gradual weeding out of the most 

 susceptible hosts. In a state of nature in 

 which medical science plays no part, there 

 must occur a slight rise in the resistance of 

 individuals, due to selection and perhaps 

 acquired immunity, which meets the de- 

 cline of virulence on the part of microbes 

 until a certain norm or equilibrium be- 

 tween the two has been established. This 

 equilibrium is different for every different 

 species of microorganism, and is disturbed 

 by any changes affecting the condition of 

 the host or the means of transmission of 

 the parasite. One result of the operation 

 of this law is the low mortality of endemic 

 as compared with epidemic diseases. Cer- 

 tain animal diseases, while confined to the 

 enzootic territory, cause only occasional, 

 sporadic disease, but as soon as they are 



carried beyond this territory, epizootics of 

 high mortality may result. Climate in 

 some eases enters as an important factor, 

 but the most important, perliaps, is the 

 slight elevation in virulence brought about 

 by a more highly resistant host. The most 

 susceptible animals are weeded out, and the 

 rest strengthened by non-fatal attacks. The 

 virulence of the microbe rises slightly to 

 maintain the equilibrium. In passing into 

 a hitherto unmolested territory, the disease 

 rises to the level of an epizootic until an 

 equilibrium has been established. 



The same is true of human diseases, 

 among which smallpox is a conspicuous 

 example. The great pandemics of in- 

 fluenza, which seem to travel from east to 

 west every one or two decades, soon give 

 away to sporadic cases, and the careful 

 work of many bacteriologists would indi- 

 cate that the influenza bacilli found at 

 present have fallen to the level of second- 

 ary invaders, and are parasites of the respi- 

 ratory tract in many affections. 



As pathogenic microorganisms differ not 

 only in the degree of parasitisni attained, 

 but also in their essential nature, a great 

 variety of diseases is the result. In a crude 

 way they may be arranged into three 

 classes : 



1. Microorganisms which live upon the 

 skin and the mucous membranes and in- 

 vade the body only when lesions exist in 

 these structures, or where the general re- 

 sistance is impaired. 



2. Microorganisms which appear only 

 occasionally from some unknown but per- 

 manent focus. They produce epidemics 

 often highly fatal, but they are success- 

 fully pushed back because the strain can 

 not readily adapt itself to the new con- 

 ditions. 



3. Microorganisms which are most highly 

 adapted for a parasitic existence and 

 which produce diseases of a relatively fixed 

 type. 



