4 INFECTION AND IMMUNITY 



fevers, yellow fever, and trichinosis. Still others, 

 such as cholera, typhoid fever, and dysentery are only 

 transmitted by personal contact under very excep- 

 tional circumstances. 



The word " infection," from the Latin verb inficio, 

 means literally to put or dip into anything ; but in its 

 accepted technical sense the putting of non-living 

 particles into a living body does not constitute in- 

 fection. A man who receives a load of bird-shot in 

 the muscles of his thigh is not infected with bird-shot 

 or with lead. But when a living micro-organism is 

 introduced into the body of a living animal and multi- 

 plies there, the animal is infected. This infection 

 may be localised, as in the case of a carbuncle, an 

 abscess, a pneumonia, a pleurisy, etc., or it may be 

 a general blood infection, as in relapsing fever, yellow 

 fever, or the malarial fevers. In cholera and dysen- 

 tery the infectious agent is in the alimentary canal, and 

 penetrates to a greater or less extent the mucous 

 membrane of the intestine. In typhoid fever it in- 

 vades the glands of the intestine and mesentery, and 

 the spleen. In diphtheria the mucous membrane of 

 the throat and posterior nares is the usual seat of the 

 infection. 



Certain infectious diseases have their seat in vari- 

 ous favourite localities upon the external portion of 

 the body. These are the infectious skin diseases, 



