736 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1909. 
for more than three or four days after it has appeared. The develop- 
ment of the state of immunity, on the other hand, is a slow process 
relatively and depends upon the setting into motion of certain cell 
functions, through which new substances are produced, which, being 
first retained within the cells producing them, eventually are passed 
into the blood. Hence it is that these new substances can be detected 
at an earlier period of the infection in the spleen than in the blood. 
But once they have been produced, the substances endure either for an 
indefinite period, or the capacity to produce new ones of the same sort 
is retained by the organism often for years. The blood may grow weak 
in the typhoid immunity principle in the course of years following an 
attack of typhoid fever, or a rabbit immunized with typhoid bacilli 
may show after a time a great diminution of the blood agglutinins for 
typhoid bacilli; but the typhoid immunity persists in the one, as in 
the other minimal quantities of typhoid bacilli will bring out, and 
without the original delay, a new production of agglutinin that will 
restore the lost amount. 
The facts on immunity which I have presented to you constitute the 
physical basis, also, of all artificial methods which are being pursued 
so successfully in preventing certain infections through vaccination, 
and in curing them through the use of immune serum products. The 
facts also account in an eminently satisfactory manner for the sup- 
pression of smallpox by cowpox vaccination. The “ vaccines” so- 
called for bacterial diseases, which are, I might say, at present being 
employed chiefly in protecting animals from epidemic infectious dis- 
eases to which they are much exposed, consist for the most part of 
bacteria either killed outright by heat or chemicals or of bacteria 
whose virulence has been diminished by special methods of cultivation 
or treatment. In human beings this method of vaccination has been 
employed only when large numbers of persons have been exposed to 
infections from the zone or focus of which they could not be removed, 
or from which, owing to the peculiar circumstances surrounding the 
infections, they could not readily or at all be protected by the suppres- 
sion of the diseased germs at their sources. Thus, it has been found 
advantageous in a few instances to employ vaccination against cholera 
and bubonic plague, on those especially exposed to these epidemic dis- 
eases, and against typhoid fever on troops going in time of war into 
heavily infected endemic zones of that disease. 
In a few instances this method of vaccination has been successfully 
carried out in animals with infectious diseases in which the germs 
causing them have not been discovered. Thus, it is possible to vac- 
cinate cattle against the destructive rinderpest of Africa, the Philip- 
pines, and other tropical countries, by employing the bile of animals 
which have succumbed to the infection, which contains the parasite 
of the disease somewhat modified by certain immunity principles 
