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 Fie. 43.—A malarial mosquito, An- 
from malaria. The organism that opheles maculipennis Say. Enlarged 
: im and natural size. Original. 
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 
