824 



SCIENCE. 



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



a certain degree supported by a phenom- 

 enon of considerable biologic importance, 

 which I wish to discuss very briefly. 



If we examine the statistics of the va- 

 rious infectious diseases we are struck Avith 

 the relatively low mortality of most of 

 them. There are few highly fatal plagues 

 now known. To be sure, the mortality of 

 many infectious diseases is regarded as 

 formidable by sanitarians, but if we dis- 

 engage ourselves from the humane view for 

 the moment and take the biologic stand- 

 point, we will agree that the relatively 

 high mortality of 25 per cent, to 50 per 

 cent, indicates a very decided preponder- 

 ance of the resisting powers of the human 

 race. The odds are always against the 

 invading microbe. This state of affairs 

 appears for the moment to contradict the 

 results of experimental bacteriology, which 

 teach us that the virulence of microbes 

 may be more or less rapidly raised by re- 

 peated passages through susceptible ani- 

 mals, or even through those which possess 

 considerable resistance. The accustoming 

 of bacteria to antiseptics, bactericidal and 

 agglutinative serums, has already been men- 

 tioned. With this capacity for adapting 

 themselves to the defensive mechanisms of 

 the host, why should not the infectious 

 diseases become more, rather than less, 

 virulent? "What is it that keeps their 

 virulence on a Ioav level? This problem 

 has occupied my attention for a number 

 of years, but only recently did a fairly sat- 

 isfactory explanation present itself. Be- 

 fore entering upon it I have still one other 

 phase of the problem to consider. 



Of a given number of races of the same 

 species of bacteria, the one which becomes 

 parasitic on a given host species is not nec- 

 essarily the most virulent for that species. 

 This phenomenon impressed itself upon me 

 during the study of a number of races of 

 the bacillus of septicemia hemorrhagica, 

 or, more familiarly, rabbit septicemia. 



Races of this species have been found very 

 widely distributed among mammals and 

 birds. Epizootics due to it have been de- 

 scribed as occurring among cattle, carabao, 

 game, swine, rabbits, guinea-pigs, fowls, 

 geese, etc. It lives in the upper air pas- 

 sages of many domestic animals in health. 



The rabbit may be successfully inoculated 

 with any of these races. Some are very 

 virulent, for the merest scratch of the skin 

 inoculated with them will result in death 

 within twenty-four hours. But the rabbit 

 is not attacked spontaneously by them, 

 although they are ubiquitous. The race 

 which has fastened itself upon the rabbit 

 is one of a very low degree of virulence 

 for that animal. Similarly the highly viru- 

 lent tubercle bacillus of cattle is encoun- 

 tered only occasionally in man, although 

 the opportunities for a transfer from cattle 

 to man are very good. 



On first thought, it would seem to us that 

 the most virulent race would be the one to 

 crowd out any less virulent races and to 

 finally predominate. But comparative 

 pathology shows us that the contrary may 

 be true. 



The explanation for these apparently 

 discordant facts readily flows from a con- 

 sideration of the life history of parasitic 

 microorganisms. This briefly consists 

 of three phases, the entry into the host, 

 the temporary multiplication therein, and 

 lastly, the escape to another host. Each 

 step in this life cycle must be carefully 

 and deliberately worked out in the evolu- 

 tion of parasitic organisms, and each de- 

 mands a special mechanism adapted to the 

 purpose. One step is as important as the 

 other. The parasite must find an un- 

 guarded entry or one which yields readily 

 to its efforts. It must have a means of 

 defense within the body and it must finally 

 reach the exterior to enter a fresh sub.ject. 



As a result of these needs, each micro- 

 organism producing disease has one or sev- 



