IMMUNITY 63 



have been known to transmit diphtheria, typhoid, 

 scarlatina, and other conditions. Flies are potent 

 carriers of typhoid by soiling their feet on excreta and 

 then alighting on food. Other insects such as mos- 

 quitoes transmit disease. It may be laid down as a law 

 that with the exception of the few infectious disorders 

 only carried by insects, intimate personal contact is 

 the most prolific source of the spread of disease. 



Bacteria are directly the cause of ptomain poisoning, 

 although the ones concerned may not live within the 

 body. Ptomain poisoning is a violent irritation of the 

 gastro-intestinal tract by certain poisons produced 

 from putrefaction of meat and fish by bacteria. The 

 foods may be little or none altered by these poisonous 

 substances in them. They are in small quantity in 

 the food, but are easily and quickly absorbed. It is 

 possible that for a short time after ingestion of the 

 meat the formation of these ptomains may continue. 

 The ptomains are toxins, but they are formed by alter- 

 ing the chemical composition of the meat rather than 

 by any peculiar products of the bacteria or poisons 

 within their bodies. The condition is not transferable. 



IMMUNITY 



The resistance offered to the entrance of micro- 

 organisms into the body has already been referred to, 

 and now the method by which our physiology gets rid 

 of the effects of these noxious agents must be con- 

 sidered. It is a well-known fact that illness does not 

 occur every time pathogenic bacteria gain a foot- 

 hold on or within the body. Sometimes a small 



