730 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1909. 
so that more individuals come under their influence; but with even 
such a common bacterium as the diplococcus which causes pneumonia 
and the bacillus which produces influenza, there arise conditions in 
which severe and often very extensive outbreaks, or localized epi- 
demics, occur which are probably to be attributed to an accession in 
virulence of these germs, although the precise causes leading to the 
increase may not be discovered. 
Now this quality of virulence, which is often evolved so quickly 
and apparently so mysteriously is expressed biologically in various 
ways besides in that of greater infective power; virulent bacteria may 
prove incapable of being charged with opsonin so that they can not 
be ingested by phagocytes; they may show unusual power to resist 
plasma or serum destruction; they may drive away or repel or act 
negatively in respect to chemical attraction on the phagocytes; and 
being thus unopposable they tend to multiply quickly and with little 
restraint and thus still further to break down and render ineffective 
> the normal defensive mechanisms, and ultimately to damage seri- 
ously the sensitive cells of the organs. This constitutes disease. 
Another power resides in the body that should be regarded, namely, 
the power to neutralize or destroy poisons as distinct from parasites; 
for the body is exposed to the deleterious action of poisons generated 
by living parasites that do not themselves penetrate within the body. 
Some of these poisons are generated away from the body, as is the 
case with certain food poisons; some by bacteria in the intestinal canal 
that do not seek to invade the blood; some by bacteria, like the diph- 
theria bacillus, that first kill tissue, usually of the mucous membranes, — 
and then develop in the dead tissue and send the poison into the body. 
And besides this, every bacterial disease resolves itself ultimately into 
a process of poisoning—of intoxication. In typhoid fever, in pneu- 
monia, 1n meningitis, and in the multitude of other bacterial invasive 
diseases of man and the lower animals, the severe symptoms are 
caused by the poisons liberated through disintegration of the invading 
bacteria, which, however, continue by multiplication to recruit their 
numbers. 
The condition of susceptibility to poisons varies with different races 
and species, very much as bacterial susceptibility does. The cold- 
blooded animals are indifferent to poisons that are very injurious to 
warm-blooded animals, but not all cold-blooded animals behave alike. 
Tetanus toxin is alike innocuous for the frog and the alligator, but by 
raising the temperature artificially the frog develops tetanus, but the 
alligator does not. Sometimes the effects depend merely upon the 
mode of entrance of the poison into the body. Tetanus toxin, diph- 
theria toxin, and snake venom have no effect on mammals when swal- 
lowed unless the intestinal epithelium has been injured. These poisons 
can not pass through the epithelium to reach the blood. where alone 
