54 INFECTION, PHAGOCYTOSIS, OPSONINS 



insoluble toxins. Likewise, in the body of an infected animal 

 such bacteria are in all probability subjected to more or less dis- 

 integration when their intracellular toxins are set free and exert 

 their harmful influence. When pathogenic bacteria invade the body 

 of an animal it may happen that they multiply enormously in a very 

 short time, as in the case of the anthrax bacillus. Again, they may 

 multiply very moderately, but evidently produce either a large amount 

 of poison or a very powerful soluble toxin. The tetanus bacillus 

 is of this latter type. Very little is known of the real intrinsic chem- 

 ical nature of most of the toxins, and they cannot be distinguished 

 by chemical tests. Still, their dissimilarity is clearly shown by their 

 different effects upon susceptible animals and by the production of 

 different signs and symptoms which made it possible to distinguish 

 many infectious diseases long before microorganisms were known. 



Virulence. Not only bacteria of different types, but also the same 

 kind of bacteria at different times, in different countries, and under 

 varying conditions, produce diseases which vary much in intensity 

 and in percentage of mortality. Bacteria which produce a violent 

 form of the disease with high mortality are called virulent bacteria 

 (the condition is called virulence or mrulency). Often an epidemic 

 starts with a low degree of virulency, gains more and more up to a 

 certain point, and then diminishes again. 



The virulence of bacteria depends more or less upon their power 

 to multiply rapidly in the infected animal or upon their ability 

 to produce a large amount of very powerful toxins. Virulency, 

 however, does not depend upon the pathogenic bacterium itself 

 alone, but also upon properties of different individuals of the infected 

 species of animal, this variable factor is known as individual suscepti- 

 bility. If animals of the same species are all exposed to one and the 

 same infectious disease, some may contract it in a very violent form, 

 other in a light form, still others not at all. This variability may depend 

 upon age, sex, special breed, color, robustness, weakness, good or 

 bad state of nutrition, or upon factors not yet recognized. 



Increase of Virulence. When pathogenic bacteria are studied ex- 

 perimentally it is generally found that their virulency can be increased 

 in the following manner: A number of animals are first inoculated 

 with a moderate dose. A certain percentage get very sick and die. 

 From one of the very sick animals the pathogenic bacterium is ob- 

 tained and again a number of animals are inoculated. This time a 

 larger percentage get very sick and die. If this procedure is con- 

 tinued a number of times an infective bacterium is obtained of very 

 great virulency which will kill all, or at least, a very high percent- 

 age. Nature does the same thing when an epidemic starts beginning 

 with a mild form of the disease and developing into a very fatal, 

 virulent form. On the other hand, at certain seasons, a fatal epidemic 

 leads successively to milder forms of the disease. 



