RESEARCHES BY PASTEUR 137 



1. All contagious diseases are caused by living bacteria, 

 which multiply in the blood or in the tissues of the invaded 

 organism or, in other words: All bacteria able to multiply 

 and to live more or less steadily in the blood or in the tissues 

 of a superior organism, may cause infectious diseases which 

 are directly or indirectly contagious. 



2. Bacteria are pathogenic because of the poisons which 

 they secrete or produce. 



3. The virulence of pathogenic bacteria may be attenuated 

 or completely suppressed in certain cases by artificial cultures 

 and may be increased by passage through living organisms. 



4. The introduction into an animal organism of a patho- 

 genic bacteria attenuated or sterilized, protects this organism 

 against later infection by the same bacteria when virulent. 

 The " vaccinated" animal thus acquires an active immunity, 

 and it is important to note that this immunity appears only 

 after an "incubation period/' which varies in time for the 

 different infectious bacteria from eight to fourteen days, and 

 which lasts for several days at least. 



Several years later, the researches of Roux and Yersin on 

 the "toxin" of diphtheria, led Behring and Kitasato to the 

 discovery of antitoxin; their researches showed that: 



5. By the injection into an animal of a bacterial poison 

 in non-pathogenic or slightly pathogenic quantity, the animal 

 acquires at first a greater resistance to the same poison, and 

 later, when the treatment is continued, there is produced in 

 the blood of the animal a specific antidote, exclusively for this 

 poison. The injection of a toxin causes the production of an 

 antitoxin in the organism. The antitoxin neutralizes in 

 vitro and in vivo the pathogenic action of the toxin. The 

 product of a mixture of toxin with antitoxin, in suitable 

 proportions, is neutral. 



These were the fundamental rules on which was based all 

 ulterior research concerning reactions between the infecting 

 bacteria and the infected organisms. It was then generally 

 thought that, on the basis of these rules, one would succeed 

 rapidly and without too many difficulties in cultivating or at 

 least in demonstrating the bacteria of all infectious diseases, 

 and in preparing preventive vaccines or curative sera for all 



