OF THB 



TVT-RSITY 



Transformation, etc., of Pathogenic Microbes. 95 



which engender the disease; the organism overcomes a 

 small dose of a certain virus when it would be over- 

 come by a larger dose : a minimum quantity of the 

 virus of gangrenous septicaemia and of symptomatic 

 charbon vaccinates against these diseases ; a larger 

 quantity produces the fatal disease. Dilution of the 

 vaccine, of the virus of sheep-pox, lessens its effects. 

 The severity of a disease may, again, be diminished 

 by introducing the virus by a special way known to 

 mitigate its influence : the blood (pleuro-pneumonia, 

 symptomatic charbon, gangrenous septicaemia, rabies), 

 the cellular tissue of the tail (pleuro-pneumonia). 



2. Preveyitive inoculation of attenuated virus. — Atten- 

 uated viruses develop a mild disease which confers on 

 the animals an immunity more solid the less the de- 

 gree of attenuation; in general, we have recourse in 

 practice to several specimens of virus of different de- 

 grees of intensity ; we begin with the weakest and 

 end with the strongest; a solid immunity may thus 

 be communicated by virus sometimes yet very active 

 but against the action of which the less virulent va- 

 rieties have fortified the organism. 



Attenuated viruses which are able to prevent the 

 development of infectious diseases are called vaccine 

 viruses or simply vaccines. 



The two charbons, rouget, pneumo-enteritis of the 

 pig, chicken cholera, rabies, etc., have furnished vac- 

 cines the employment of which is to-day admitted 

 into general practice. 



3. Preventive inoculation . of soluble vaccinating sub- 

 stances. — We have seen above, in connection with the 

 subject of acquired immunity, that the latter results 

 from the impregnation of the organism with the solu- 



