4 INSECTS AND HUMAN WELFARE 



which is the disease-producing organism, is present in the 

 dejecta of an infected person and may find its way from 

 these to food, carried by flies or otherwise; ingested by a 

 healthy person, it may quite likely multiply and induce a 

 second case of typhoid. With tuberculosis, the tubercle 

 bacillus from desiccated sputum may enter the lungs of a 

 healthy person with dust and there reproduce the disease. 

 As we shall see later, certain insects are commonly associated 

 with the spread of diseases of this type, although from the 

 very nature of such diseases, insects are not exclusive factors, 

 and may be referred to as contaminative carriers. 

 * A second type of communicable diseases differs from the 

 one just mentioned in that the organism which causes the 

 disease must live for a time in the body of some other animal 

 to undergo certain definite changes before it can again induce 

 the disease in another individual. The most important in- 

 sect-borne diseases belong to this type, for in the case of man 

 and domestic animals, certain insects and ticks act as the 

 secondary host animals for the organisms of many diseases. 

 Thus, yellow fever is spread only through the agency of a 

 certain mosquito, for in its body alone can the yellow fever 

 organism live and undergo the changes that are necessary 

 before it can be introduced into another patient by the bite 

 of an infected mosquito. Malaria belongs to the same cate- 

 gory, for it spreads only through the bite of certain mos- 

 quitoes that obtain the organisms with their meal of blood, 

 and then afterwards inject into the blood of another person, 

 a later stage of the malarial parasite which has developed 

 meanwhile within the mosquito. 



The importance of insects as detrimental to public health 

 is well known to professional zoologists, medical men, and 

 laymen alike, but is usually emphasized only under the stress 

 of particular circumstances, such as the safety of soldiers in 

 the recent war, or of unusual outbreaks of diseases for which 

 insects are directly responsible. 



Insect-borne diseases present a constant menace to the 

 world, and aside from the actual toll of lives which they exact, 



