228 BACTERIOLOGY 



a number of years, but only recently did a fairly satisfactory ex- 

 planation present itself. Before 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 neces- 

 sarily the most virulent for that species. This phenomenon im- 

 pressed 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 dis 

 tributed among mammals and birds. Epizootics due to it have 

 been described as occurring among cattle, carabao, game, swine, 

 rabbits, guinea-pigs, fowls, geese, etc. It lives in the upper air- 

 passages 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 inocu- 

 lated 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 virulent tubercle bacillus of cattle is encountered 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 finally to 

 predominate. But comparative pathology shows us that the con- 

 trary may be true. 



The explanation for these apparently discordant facts readily 

 flows from a consideration of the life-history of parasitic micro- 

 organisms. 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 evolution of parasitic organisms, 

 and each demands a special mechanism adapted to the purpose. 

 One step is as important as the other. The parasite must find an 

 unguarded 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 subject. 



As a result of these needs, each micro-organism producing disease 

 has one or several avenues of entry and escape. In some of the 

 protozoa there is but one avenue, and this is highly specialized and 

 is through the body of some insect. Among the bacteria the chan- 

 nels of escape are less highly developed, and there may be several. 

 As a rule, the microbe adapts itself eventually to a locus more or 

 less in direct contact with the exterior, and in some instances the 

 loci of entry, multiplication, and exit have coincided. If we think 



