THIRTEENTH REPORT. 
09 
It was not until 1898 that the far-reaching discovery was made that 
a human disease, namely malaria, was transmitted bv the bite of a 
certain kind of mosquito. It was indeed a revolutionary discovery 
which relegated the time-honored causes, such as effluvia, miasms, and 
bad odors to the junk-pile of exploded theories and substituted for these 
demonstrable biological facts. The unveiling of the mystery surrounding 
malaria resulted in a new and clearer conception as to the transmission 
of certain diseases, particularly those in which the germ is present in 
the blood of the diseased animal. The fact is now fully appreciated that 
insects and the like which feed upon the blood of such diseased animals 
may become infected; in other words, they furnish suitable soil for 
the growth of the germ. It must not be inferred, however, that every 
kind of blood-sucking organism which feeds upon a given animal will 
constitute a host for the parasite. There are just as great differences 
in susceptibility among the insects and arthropods as there are among 
the higher animals. The fact that man may contract a disease or be- 
come jioisoned by eating certain kind of food does not mean that the 
lower animals will respond in a like manner. Nor does it follow that 
a disease of the horse or cow is transmissible to amphibians or birds. 
The common mosquito ( Culex ) for example does not transmit malaria 
for the reason that the organism of this disease cannot pass through 
the necessary development and multiplication in this insect. A special 
mosquito of another genus ( Anopheles ) does afford suitable conditions 
for the growth of the germ and hence becomes the carrier. A further 
illustration is afforded by yellow fever which is not transmitted by either 
one of the two mosquitoes just mentioned but has its own special host 
( gtegomyia calopus ) . 
PASSIVE AND ACTIVE CARRIERS. 
It has been customary for some years to speak of insects as passive 
and as active carriers, and these terms usually convey with them cer- 
tain well-defined conceptions. The passive carrier may be looked upon 
as an accidental conveyer rather than as a living host for the organism. 
A fly feeding upon typhoid excreta may soil its feet or proboscis and 
on alighting elsewhere may deposit such mechanically adhering parti- 
cles. The part played by the passive carrier is merely one of bring- 
ing about indirect contamination. This transference of disease organ- 
isms to various articles of food or even into wounds, by flying insects 
may lead to infection and it is generally recognized that certain bacterial 
and even protozoal diseases may thus be spread. Reference need only be 
made to cholera, dysentery, tuberculosis, and typhoid fever. 
The passive carrier is by no means as important a factor in the spread 
of disease as is the active carrier. Indeed, it cannot be compared with 
the latter for the reason that the active carrier usually represents the 
only natural way by which certain diseases are transmitted. The in- 
numerable researches of the past few years all go to demonstrate the 
preponderating role played by the active carrier. Every day, so to 
speak, brings new and positive evidence regarding the transmission of 
this or that disease by active hosts, and for that reason this factor is 
deserving of special attention. 
An active carrier is essentially a diseased individual. The insect, 
