124 MICROBES AND TOXINS 



by bacteria of which he is himself a carrier. The term of 

 auto infection gained a precise signification only when germ- 

 carriers became known. 



Germ-carriers are those individuals who harbour the microbe 

 of the disease yet present no symptom of this. The fact is not 

 entirely novel (it has been known for long that the pneumo- 

 coccus exists in individuals recovered from pneumonia, and 

 Pasteur discovered it in the saliva of a child dead of rabies), 

 but it is only during recent years that all its importance has 

 been recognised. 



There exist carriers of typhoid and paratyphoid bacilli, of 

 diphtheria, of cholera, of dysentery and of meningococcus 

 There are individuals who have acquired the microbe without 

 yet contracting the disease — precocious carriers : others recently 

 cured and not yet freed from their bacteria — convalescent 

 carriers ; individuals cured weeks, months, or even years before — 

 chronic carriers : finally there are occasionally individuals who 

 have never had the disease — healthy carriers or " paradoxical 

 carriers." It is of great importance to recognise germ carriers 

 especially in communities where the general life is intimate and 

 confined, as in schools and barracks. 



In typhoid fever in particular the germ-carriers are most 

 often women. According to Frosch's statistics, there is one 

 woman for every five cases of typhoid, but of every five chronic 

 typhoid-carriers four are women. The typhoid bacilli in 

 chronic typhoid-carriers select as their favourite habitat the 

 bile-ducts. 



Conditions of Infection.— These are very variable, 

 varying with the virulence, the resistance of the body, and the 

 method of inoculation. 



The number of attacking bacteria can be calculated fairly 

 closely in experiments, but in the natural disease it is impossible 

 to tell how many bacilli are necessary to determine a given 

 infection. 



The rabbit is extremely sensitive to the bacilli of fowl- 

 cholera. Now, according to Watson Cheyne, if 10,000 to 

 30,000 bacteria are injected there is only a local abcess ; above 



