Immunity 
130 BACTERIOLOGY. 
lowing, anti spasm of the muscles of respiration. The 
spasms are very painful and may be induced even by 
the sight of water. This is the origin of the name 
hydrophobia, which means fear of water. 
All efforts to find the cause of the infection in the 
brain and spinal cord have been fruitless. Peculiar 
bodies, called Negri bodies, are quite constantly found 
in the brain and spinal cord, which many believe are 
parasites belonging to the animal kingdom, and classed 
as protozoa. The diagnosis of rabies can be made 
either by finding the Negri bodies or by reproducing 
the disease in rabbits by inoculating them in the brain 
with portions of the spinal cord of rabid animals. 
It is due to the studies of Pasteur that we are able 
to immunize against rabies. He found that the virus 
of rabid dogs could be intensified by inoculating a 
series of rabbits until the inoculation period could be 
shortened to six or seven days. The spinal cords of 
rabbits inoculated in this way contain the virus in its 
most concentrated form, and is spoken of as the fixed 
virus. 
The fixed virus may be attenuated by drying 
the spinal cords and, if human beings are now inocu¬ 
lated with portions of the tissue, beginning first with 
the most attenuated and then with more and more 
virulent tissue, an active immunity is established. 
This is the method now in use in the treatment of per¬ 
sons who have been bitten by rabid dogs, and it can 
be applied during the forty- to sixty- day incubation 
period. It has proven very successful. In the last ten 
