MAN AS A CARRIER OF DISEASE 343 



Thus the little hard-skinned parasitic thread-worms which 

 are often brought in by uncooked food into man's intestine, 

 though by themselves comparatively harmless, scratch the 

 soft lining of the bowel and enable poison-making microbes 

 to enter the deeper tissues, and cause dangerous abscesses 

 and appendicitis. 



The carriers of disease germs thus become a very im- 

 portant subject of study. There are carriers which make 

 no selection, but are, so to speak, " casual " in their 

 proceedings, and there are others which have the most 

 special and elaborate relations to some one kind of disease- 

 causing microbe for which alone they are responsible, and 

 to the life of which they are necessary. Let us look first 

 at the more casual group. Man himself is a great carrier 

 and distributor of his own diseases. Unless and until he 

 has learned to be careful and guard against thoughtless 

 proceedings, he is always spreading the microbes of his 

 diseases and passing them on to his fellow men. He pol- 

 lutes the waters, rivers, lakes and pools from which others 

 drink. He manures his crops, and then eats some of 

 them uncooked. His hands are polluted by disease- 

 causing microbes, and he handles (to an alarming and 

 unnecessary extent) the food, such as bread and fruit, 

 which is swallowed by his fellows, without cleansing it by 

 heat. It has lately been shown that apparently healthy 

 men an^ women often harbour within them the microbes 

 of typhoid fever or of cholera (and probably other diseases), 

 without themselves suffering in health, and that unsus- 

 pected they thus become distributing centres of these 

 diseases. The names " typhoid carrier " and " cholera 

 carrier" have actually been introduced to describe the condi- 

 tion of such persons. Then, again, by 4iis breath, and by 

 coughing and spitting, a man acts as a carrier to others 

 of disease-microbes already lodged in him, as well as by 

 actual contact in the case of those infections which are 



