INFECTION AND IMMUNITY 383 



was so hopelessly mixed with other pathogenic micro- 

 organisms of the saliva as to make it impossible to use 

 it as the basis of exact experiments. Looking for the 

 microbe of rabies, he found it present in greatest 

 intensity in the nervous system, and found that by 

 rubbing up the nervous tissue from the brain or spinal 

 cord with physiological salt solution, it was possible to 

 secure the virus in a form free from admixture with other 

 microbes and convenient for experimental investigation. 



Unfortunately, though there was every evidence that 

 the disease was infectious and therefore microbic, he 

 found it impossible either to demonstrate the specific 

 microbe by the microscope or to make it grow in artifi- 

 cial culture. In regard to this it may be well to remark 

 that we are not yet able to cultivate this microbe, though 

 there seems to be little doubt about it being a protozoan 

 parasite discovered by an Italian named Negri. 



Disregarding his inability to demonstrate the microbe 

 and finding that he was perfectly able to reproduce the 

 disease experimentally, by using the emulsion of the 

 nervous tissues, Pasteur set about finding methods of 

 attenuating the virus. The results were most interest- 

 ing. The nervous tissues of dogs and other animals with 

 the disease were found to yield viruses of varying degrees 

 of virulence, "street virus"; but after such a virus was 

 manipulated in the laboratory by passage through a 

 series of rabbits, it acquired a uniform degree of virulence 

 and became known as a "fixed virus." Such a fixed 

 virus, contained in the spinal cord of a rabbit, was 

 further found to be susceptible of attenuation by drying, 

 the degree of attenuation being proportionate to the 

 length of the period of drying. It was not difficult to 

 arrive at any degree of attenuation until virulence was 

 eventually entirely lost. By working with the attenu- 

 ated viruses, and administering a succession of doses 

 with increasing degrees of virulence, animals could be 

 immunized against the virulent " street virus. " 



As, of course, no one would desire to be immunized 



