MICHIGAN ACADEMY OF SCIENCE. 
21 
is eliminated from the kidneys for weeks, and even months after recov- 
ery and hence man as well as the goat becomes a carrier. In cholera, 
dysentery and other bacterial diseases this same problem of carriers is 
one that claims attention but it is not my purpose to do more than 
point out this fact. 
But carriers are not restricted to the bacterial diseases which have 
been mentioned. They play an even more important part in the propaga- 
tion of certain protozoal infections. They constitute the natural reser- 
voirs of virus and, as such are chiefly responsible for the continued 
existence of such diseases. Thus, cattle which have recovered from 
Texas fever do not show on microscopical examination of their blood 
any evidence of the presence of the parasite, and yet such blood when 
injected into a healthy animal gives rise to the typical disease. The 
parasite is present in an unrecognizable form in the immune animal ; 
a fact which implies that a reciprocal immunity has been established, 
an armed truce, so to speak, between the host and the invader. 
In spirochete and trypanosomal diseases carriers are again in evi- 
dence. Uere also the organisms, though present in the blood are un- 
recognizable by means other than inoculation of animals and exception- 
ally by artificial cultivation. It would be interesting to know just 
what form is assumed by these parasites which enables them to escape 
direct detection. Some indication of this transformation is already at 
hand and will be referred to presently. 
The disease caused by sub-microscopic or invisible organisms may show 
this same persistence of the infective agent long after recovery has 
taken place. A striking example of this fact has but recently been de- 
termined in connection with infantile paralysis. It has been found that 
the virus persists in the naso-pharynx for many months. Under such 
circumstances it is not surprising to learn of sudden outbreaks of this 
dangerous disease in a locality where no previous case was known to 
exist. 
INSECT CARRIERS. 
There remains another type of carrier, other than man or the ordinary 
animal, namely the insect or arthropod or other sanguivorous host. It 
may be said at once, and without any fear of contradiction, that the 
greatest achievements, the most valuable results which have been ob- 
tained during the past 10 or 12 years have come from the recognition 
of the important part played by these carriers in the transmission of 
disease. There were, indeed, years ago, prophets who pointed out the 
probability that malaria and yellow fever, were mosquito borne diseases 
but the experimental proof was not furnished until quite recently. 
The early bacteriologist could see only three ways for the trans- 
mission of disease, namely inoculation through wounds, inhalation of 
dust particles, and infection of food and water. That seemingly in- 
significant bites of insects could result in the production of serious 
disease did not appear reasonable and for that reason when the American 
workers in 1891 pointed out that Texas fever was caused by the bite 
of a tick, and further, that the tick in question had never been in con- 
tact with a diseased animal, but had developed from eggs laid by its 
blood sucking parent, it was not surprising that open scepticism if not 
actual ridicule was bestowed upon the work. 
