94 INFECTION AND IMMUNITY. 



may produce infection when given intraperito- 

 neally into the guinea-pig, and twenty to thirty 

 when into the rabbit (Wyssokowicz). Many hun- 

 dreds of the most virulent cholera vibrios and 

 typhoid bacilli are required to produce fatal infec- 

 tion in the guinea-pig. In contrast to the condi- 

 tions mentioned are various saprophytic organisms 

 which, regardless of the quantity introduced, either 

 do not produce infection at all, or do so only after 

 the resistance of the animal has been lowered arti- 

 ficially. 



Among those who are equally exposed to infec- 

 tion in an epidemic of typhoid fever, the escape of 

 many probably is due to the ingestion of a small 

 quantity of bacilli which is insufficient to produce 

 disease. Individual susceptibility, and temporary 

 low resistance, are other factors. 



(2) Types of Infection. 



There are wide variations in the physical rela- 

 tionships which different pathogenic micro-organ- 

 isms hold to the tissues of the body. This has 

 already been suggested in the discussion of infec- 

 tion atria, in which it was shown that certain 

 organisms have a specific preference for points of 

 primary invasion. 



This tendency of a specific relationship to par- 

 ticular tissues is kept up in many instances after 

 the microbes have reached the interior of the body. 

 selective The malarial plasmodia enter and destroy the 

 erythrocytes and cause enlargement of the spleen 

 and liver, while other organs are affected to a 

 much less degree as a rule. The pneumococcus 

 has a great affinity for the pulmonary tissue and 

 for endothelial structures; it frequently causes 



