CHAP. XX. INSEX TS AND DISE 183 



(sporozoites) which make their waj into the body-cavit) of the 

 mosquito and thi i salivary glands whence they may be 



< arried with the sain ary fluid into the body of a human being bitten 

 by the mosquito. Thi upied bj the development of the 



parasite in the mosquito is from eight to twelve days or more-, 

 whence it follows that a mosquito which has become infected by 

 biting a malarial patient is not itself infective and cannot transmit 

 the parasite until after this interval of time, which is practically not 

 before the tenth night after having ingested the par isiti But, after 

 this, one infected mosquito may bite and infect more than one subject 

 during i if its life. 



In comparison with the above method of infection maj be noted 

 . of a House-fly which first settles and feeds on excrementi- 

 tious matter contaminated with the " germs " of Typhoid Fever and 

 then flies into a house and crawls over food which it infects with 

 the germs carried on its feet and in its alimentary canal, with the 

 result that those eating the food afterwards ingest the germs anil 

 may develop Typhoid Fever. In this case the fly carries the infec- 

 tion only more or less casually and the parasite does not undergo 

 anj special cycle ol development on or in the carrier nor is the 

 parasite entirely as a means of access into 



the human body, as it may also be carried by eating or drinking 

 food which has been contaminated din 



It is convenient to differentiate between thes< • s, that 



es in which the parasite causing the disease in man or animals 

 is intimately adapted to the insect-carrier in which it undergoes 

 part of its life-cycle, and cases in which the parasite is merely 

 carried more or less mechanically by the insect-carrier from one 

 host to another. In the former cast- the insei t-carricr is a true or 

 adapted carrier, in the latter case it is a more >>r less casual porter. 

 It will readily be understood how easily a blood-living ; 



rried directly from one host to another by transfer on the 

 mouthparts of a blood-sucking insect which is disturbed from one 

 il at once proceeds to bite another of the same kind of 

 animal. The term "M .reek miastor, a carrier of pollu- 



tion) has been proposed bj Alcock as a term applicable to 

 which by mainly mechanical means spread abroad any kindol 



lis," under which term we ma) include not only 

 bai teria but org.: testinal worms. 



'! and beast 



many insects and similar animals ma) cause direct annoyance or 



b\ their 5, poisonous hairs or spines or similar 



on of tin ■ the body. The 



irritation caused g insects, such as 



