IMMUNITY. 257 



the year when the Hindoos flock to the Ganges to worship 

 they live in crowded camps, and under these circumstances 

 cases of cholera are always very numerous. 



Pettenkofer believes that the disease is never transmitted 

 from one individual to another, but that the germ must first 

 mature in the earth for a certain length of time before infec- 

 tion can occur. He also believes that infection occurs through 

 the respiratory tract. His theory is known as the ground- 

 water theory. 



Garden vegetables sprinkled with water containing the 

 cholera germ ; and milk which is either diluted with or con- 

 tained in cans that have been washed in infected water, may 



FIG. 111. 



Stab cultures of three cholera spirilla in gelatin, showing in upper portion of 

 growth considerable liquefaction of nutrient gelatin. (Park.) 



be the source of infection. A number of investigators have 

 become infected with pure cultures of cholera while conduct- 

 ing experiments in the laboratory on animals. 



Immunity : The serum of the blood of animals that have 

 recovered from cholera contains a substance which has decided 

 bactericidal properties. This substance is not an antitoxin. 

 The immunity is entirely dependent upon lysogenicity, or the 

 formation of lysogenic bodies in the blood which possess a 



17 Bact. 



