TRANSMISSION OF DISEASE 



43 



ment later comes through our opened doors or unscreened windows, 

 its hairy feet loaded with dangerous germs, and alights on the food set 

 on our dinner table. Or, coming from the street, where it has been 

 feeding on the sputum of some unfortunate victim of tuberculosis, 

 it brings in the deadly bacteria in the ridges and hollows of its tongue. 



Mosquitoes 



In a wholly different manner the mosquitoes of certain species have 

 been proved to transmit malarial fever. Indeed it is known that this 



disease never is transmitted in 



any other way. Here, in contrast 



to the fly which simply carries 



germs mechanically on some part 



of its body, we have an insect 



that serves as an intermediary host 



to the organism, the latter going 



through a definite part of its life 



round within the body of the 



insect, the remainder within the 



body of man. The mosquito 



itself is infected by sucking the 



blood of a human being suffering 



from malaria. The organism that 



causes the disease, being thus 



transferred to the stomach of the insect, goes through certain changes, 



and eventually collects in large numbers in the salivary gland of the 



mosquito. If, now, this insect bites another person, the organisms are 



transferred to the latter, and shortly develop in the blood, giving 



rise to the characteristic chills and fever, recurring at regular intervals, 



according to the particular type of organism with which the mosquito 



has been infected. 



As a direct result of this knowledge it has been possible to bring 

 about phenomenal results in fever-ridden districts, by careful screening, 

 and by isolating fever patients so that mosquitoes could not get at 

 them while they were suffering from the disease. In places where this 



Fig. 43. — A malarial mosquito, An- 

 opheles maculipennis Say. Enlarged 

 and natural size. Original. 



