IMMUNITY TO ORGANISMS 243 



The mere presence of infective and invasive bacteria 

 upon a mucous surface is, again, not tantamount to infec- 

 tion. In every epidemic disease, we know or have reason 

 for believing that many more persons carry the germ of 

 the disease than actually contract it. Thus diphtheria 

 bacilli and meningococci are found during an epidemic in 

 the throat and naso-pharynx of many well persons who 

 never develop the disease, and similarly during prevalence 

 of cholera, dysentery, and enteric fevers, the bacilli causing 

 them are present in the intestinal tract of persons in 

 health. And these healthy " carriers," while they them- 

 selves may escape infection, are frequently the means of 

 infecting others. The reason for the phenomenon is to 

 be sought in an adequate defensive mechanism in the one 

 group who escape infection, and in a defective mechanism 

 in the other acquiring it. That this is the explanation is 

 suggested by the fact that in the case of a cholera carrier 

 the ingestion of irritating substances may transform the 

 well carrier into a case of cholera. 



Infants frequently exhibit a more marked resistance to 

 some of the diseases of childhood, e.g. measles, than do 

 older children. This may be, and probably is, due to the 

 transference of protective substances from the mother 

 to the child first by the placental circulation and later 

 by the milk. The immunity being passive (p. 252), tends 

 to disappear, so that the child of a few years becomes 

 susceptible. 



Not all individual parasites of the same species, 

 whether bacterial, protozoal or ultramicroscopic, are 

 potentially equal as agents of infection. The quality 

 of virulence, so called, is of high importance. Not a 

 few of the common parasites vary greatly in virulence, 

 i.e. in capacity for infecting, from degrees that make them 

 almost harmless to degrees that make them inconceivably 

 potent. This state of virulence in some instances is 



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