94 SAPROPHYTISM, PARASITISM AND PATHOGENISM 
{a) Direct Contact.— The transfer of glanders from the horse to 
man, of anthrax from cattle and sheep to man and of hydrophobia 
from dogs to man represents direct transfer of the virus from the 
sick animal to the well man. Other diseases are thus transmitted, 
but the examples given suffice for illustration. It should be empha- 
sized that both man and the animal are susceptible to the micro- 
organism in such cases. 
(b) Indirect Transfer.— Insects are common carriers in the indirect 
transmission of the virus of disease from man to man. Flies are 
knowm to have carried typhoid bacilli from typhoid dejecta to milk 
or other food, which in turn has been consumed by man, resulting in 
infection. The same insect, doubtless, when conditions are favorable, 
can and does carry other enteric bacteria — paratyphoid, dysentery 
and even cholera organisms. It is very probable that other insects 
also participate in the indirect transmission of bacteria. Acute 
conjunctivitis, particularly that form which is prevalent in Egypt, 
is supposed to be spread in this manner. The indirect transfer of 
pathogenic bacteria is, of course, purely accidental, insofar as the 
part played by the insect is concerned. The disease would probably 
spread if all insects were eliminated. 
(c) Mechanical Transf er.Suctor'isd insects are known to transmit 
the viruses of certain diseases which circulate in the blood stream of 
animals to man, incidental to feeding. Thus, the flea transmits the 
plague bacillus from rat to man, from man to man, and possibly 
from man to the rat. The louse similarly spreads the virus of typhus 
from man to man. In the instances cited the insect is probably not 
a true intermediary host, for the virus does not necessarily multiply 
in the insect, nor does the virus undergo any essential transformation, 
so far as is known, in the insect. Nevertheless, the transmission of 
the viruses of these diseases —bubonic plague and typhus, for example 
— depends upon the agency of suctorial insects for their passage from 
hose to host. Other insects also transmit disease, but the evidence in 
a majority of instances is somewhat less definite than that of the cases 
cited. 
(d) Intermediary Hosts. — Certain insects, notably mosquitoes, 
transmit disease from man to man only after the virus has passed 
an extracorporeal cycle in the extrinsic host— the mosquito in this 
instance. Thus, Anopheles transmits malaria from man to man and 
Stegomyia fasciata, or as it is now called, Aedes cegypti, transmits 
presumably in a similar manner, the virus of yellow fever. Trans- 
mission in these cases is through the female insect and a definite 
interval (latent period) must elapse between the time of biting the 
patient and the time when the mosquito becomes infect Iac to the 
non-immune host. 
6. Human Carriers.— Individuals who are apparently healthy 
occasionally harbor within their bodies (in free communication with 
the exterior, however, either through the respiratory tract, the gastro- 
