828 



SCIENCE. 



[N. S. Vol. XX. No. 520. 



As regai'ds lie iirst class, the conditions 

 under which they prodnce disease rise more 

 and more iato prominence. The factor mi- 

 crobe becomes almost secondary to other 

 factors. Many of our most common dis- 

 eases obey certain meteorologic laws. Thus 

 diphtheria and pneumonia are chiefly win- 

 ter diseases, because the conditions of 

 throat and lungs which favor them are 

 largely due to cold weather, or we might 

 say, the cold weather acting upon an indoor 

 sedentary population or one subjected to 

 untoward influences, injures the respira- 

 tory tract. Some microbes of this class 

 depend upon the preparation made for 

 them by others. Thus the exanthematous 

 diseases, such as scarlatina and smallpox, 

 are frequently associated with or followed 

 by the invasion of streptococci, and the 

 majority of deaths are due to such secon- 

 dary invasion. The streptococci live upon 

 the Tnncous membranes, and Tvheiiever the 

 proper opportunity comes they invade 

 jnore vital territory. This group of bac- 

 teria is the frequent cause of death in 

 chronic diseases. Some years ago Pro- 

 fessor Flexner pointed this out and denom- 

 inated the invasion as a terminal infection. 

 I think that they may also be appropriately 

 styled the parasites of the diseased state. 



Among the second group we may place 

 such diseases as Asiatic cholera and the 

 bubonic plague. The origin of the first is 

 unknown. The definitive host of the second 

 is probably the rat. 



Among the third class we have such 

 groups of diseases as tuberculosis, leprosy, 

 syphilis and glanders, on the one hand, and 

 the eruptive diseases, on the other. The 

 former are very chronic, protracted, the 

 widely separated but highly parasitic 

 latter acute, rapid in their course. In the 

 eruptive -'diseases the infection seems to 

 depend solely upon the specific suscepti- 

 bility of the individual, and immunity is 



easily brought about by protective inocu- 

 lation. 



In tuberculosis and leprosy the mode of 

 infection is evidently very diiferent from 

 that of the group just mentioned. Pro- 

 longed exposure, as in family life, seems 

 necessary to successful infection, and even 

 then many exposed individuals escape. In 

 tuberculosis, heredity plays a very promi- 

 nent part in the eyes of the physician, 

 because the disease appeared to propagate 

 itself in families. This was probably due 

 to the necessity for more intimate associa- 

 tion and repeated exposure in order that 

 the disease might appear. Here the disease 

 is long drawn out, the parasite may become 

 in a sense individualized, and the attack 

 upon a new host may have to be made re- 

 peatedly. With these highly parasitic 

 forms the necessity for a frequent transfer 

 to another host is slight. In leprosy, the 

 disease may last fifteen years to twenty 

 years, and then death ensues, usually as a 

 result of the attack of the secondary in- 

 vaders. 



From the biologic standpoint which I 

 have endeavored to present, we may con- 

 ceive of all highly pathogenic bacteria as 

 incompletely adapted parasites, or para- 

 sites which have escaped from their custom- 

 ary environment into another in which 

 they are struggling to adapt themselves, 

 and to establish some equilibrium between 

 themselves and their host. The less com- 

 plete the adaptation, the more virulent the 

 disease produced. The final outcome is a 

 harmless parasitism or some well-estab- 

 lished disease of little or no fatality, unless 

 other parasites complicate the invasion. 

 The logical inference to be drawn from the 

 theory of a slowly progressive parasitism 

 would be that in the long run mortality 

 from infectious diseases would be greatly 

 reduced through the operation of natural 

 causes. But morbidity would not be di- 

 minished, possibly greatly increased, by the 



