SOURCES OF INFECTION 37 



throat and some others, occur because persons or animals suffering 

 with the disease have handled or supplied the milk. This direct 

 contamination also applies to food like meat and oysters (typhoid 

 and meat poisoning). 



Soil-borne diseases are chiefly those arising by direct implanta- 

 tion of earth into the body. Of course the ground may be soiled 

 by discharges from infectious diseases and contamination of 

 hands and clothing. 



Animal carriers of disease include those acting as intermediate 

 hosts (anopheles mosquito in malaria) ; as mechanical conveyances 

 of a direct or indirect nature, in the former case like transmission 

 of organisms from a sick man or animal to a well one, in the latter 

 case transferring the germs to food consumed by a healthy being; 

 or acting as a passive host for the germ as is the case in the 

 transmission of plague bacilli by the rat flea. 



Human transmission of infective matter is the most important 

 of all methods as it is an axiom that a person suffering with a 

 disease is most capable of transmitting it. This occurs by direct 

 contact, by the carrier state and by passing the contagium to the 

 embryo. 



Carriers. After recovery from certain diseases, notably ty- 

 phoid fever, diphtheria and cholera, convalescents may carry 

 in themselves fully virulent germs with no outward evidences 

 thereof. Such persons are called " carriers" and are of the highest 

 importance in hygiene. The reasons for this condition are several. 

 These germs may be removed from the bodily defenses or the 

 body may be immune to them; again they may be fixed or fast 

 strains. Wherever they are they may escape and infect another 

 person. After typhoid fever bacilli remain in the gall-passages 

 and bladder; after cholera in the deep mucous membranes and 

 after diphtheria the crypts of the tonsils or the nasopharynx may 

 hold them. Vaccination or operation may be needed to remove 

 them. Persons never known to have had enteric fever have 

 been known to harbor bacilli in their gall-bladder. One typhoid 



