I. \IMf\IZATION AGAINST HYDROPHOBIA. 81 



small to warrant a conclusion. It seems quite certain, however, 

 that vaccine therapy may prove of value in this disease. It must 

 l.c lionu- in mind that there are different strains which can produce 

 the disease and that an autogenous vaccine will most likely give 

 the best results. Usually fifty million bacilli are injected, the inter- 

 val between injections being from eight to ten days. 



Cholera Vaccines. Vaccines made from cholera vibrions have 

 been used in protective immunization. Statistics on this method 

 are still too meagre to warrant definite conclusions on its value. 



Less common Infections. Vaccine treatment has been ap- 

 plied in most all infections produced by bacteria. The results 

 obtained' have varied, though Wright believes that the method 

 is applicable in the treatment of all infections caused by bacteria. 



SPECIFIC VACCINES FOR DISEASES OF 

 UNKNOWN ETIOLOGY. 



Rabies, Hydrophobia or Lyssa. This is a disease whose eti- 

 ology is only indefinitely established. The organism responsible 

 for the condition has never been grown. By some investigators, 

 certain peculiar bodies, found in 1903 by Negri in the large nerve 

 cells in the central nervous system, are regarded as the causal factors 

 in the disease. Although these bodies are not universally accepted 

 as the etiological factors, they are quite generally accepted as spe- 

 cific to this disease. 



The inability to establish the etiology has not prevented the 

 establishment of methods of immunization to this disease. The 

 principle of the method of immunization most generally practised 

 is" based on the establishment of an active immunity during the 

 period of incubation of the disease. In man the period of incuba- 

 tion usually lasts from four to six weeks, though it has been found 

 to be as short as fifteen days and as long as one year. The rela- 

 tively long period of incubation makes it possible to establish an 

 active immunity before the symptoms of the disease develop. 



Immunization is accomplished by repeated injections of so- 

 called "fixed virus." Pasteur found that tissues and fluids from 

 rabid animal vary considerably in virulence, and that the viru- 

 lence of virus can be changed considerably. If successive re-inocu- 

 lations into rabbits be made from the virus of a rabid dog the virus 



