494 
THE BIOLOGY OF DISEASE 
The mosquito sucks the blood from a man or an animal 
suffering from malaria. This blood contains some of the 
malarial parasites, which pass into the stomach of the mos¬ 
quito. They then migrate into the salivary glands of the 
mosquito, so that as soon as the mosquito bites another 
man or animal, it pours out some saliva which introduces 
the parasites into the victim’s blood. While in the body 
of the mosquito, these parasites pass through definite stages 
in their life history; and when they reach the blood of man, 
the remaining stages are completed. Thus a man, or an 
animal, and a particular mosquito are necessary for the 
complete life history of the malarial parasite. 
This means in addition that for the prevention of malaria 
all that is necessary is to prevent the Anopheles mosquito 
from breeding, or in case this cannot be done, to screen ade¬ 
quately the houses, tents, and bedrooms in the regions where 
the mosquitoes live. It is interesting to note that the 
methods for the prevention of malaria were more than 
anything else responsible for the successful completion of 
the Panama Canal. The construction of this important work 
thus became a health as well as an engineering problem. 
380. Other Protozoan Diseases. — Other protozoan dis¬ 
eases are produced in the same manner as malaria. The 
carrier may be different, but the principle of spreading the 
diseases is the same. Yellow fever, for instance, is spread 
by another kind of mosquito, and sleeping sickness by the 
tsetse (tset'se) fly. (See page 55.) 
381. Hookworm Disease. — This disease is caused by a 
parasite which is classified as one of the worms. Hook¬ 
worm disease belts the earth in a zone which extends thirty- 
three degrees each side of the equator. Great progress is 
being made in the United States in curing those suffering 
from this disease. The wearing of shoes and the use of a 
sanitary closet are usually sufficient preventives to protect 
the people who live in a hookworm district. 
