IMMUNITY. 103 
is reduced by disease and the amount of infection is 
great. The inability of a micro-organism to grow 
in the body of an animal does not usually indicate, 
however, an insusceptibility to its poison; thus, for in- 
stance, rabbits are less susceptible than dogs to the effects 
of the poison elaborated by the pneumococci, but these 
bacteria develop much better in the former than in 
the latter. Differences in susceptibility are sometimes 
very marked among different varieties of the same race 
of animals, as, for instance, between different kinds of 
rats and pigeons to anthrax. In animals, as a whole, 
it is noticed experimentally that the young of all species 
are less resistant to infection than the older and larger 
ones. 
The difficulty experienced by the large majority of 
bacteria in developing in the tissues of the healthy 
body can be to a great extent removed by any cause 
which lowers the general or local vitality of the tissues. 
Among the causes which bring about such lessened re- 
sistance of the body are hunger and starvation, bad 
hygienic surroundings, exhaustion from overexertion, 
exposure to cold, the deleterious effects of poisons, bac- 
terial or other, acute and chronic diseases, vicious 
habits, drunkenness, etc. Purely local injuries, such 
as wounds, contusions, etc., also give sometimes a point 
of entrance for infection, or at least a point of less re- 
sistance, where the bacteria may develop and produce 
local inflammation. This is noted in infection by the 
tubercle and typhoid bacilli, pyogenic cocci, etc. 
Local affections, such as endocarditis, may also afford 
a weak spot for the bacteria to seize upon. The pres- 
ence of foreign bodies in the tissues in like manner 
predisposes them to bacterial invasion. Interference 
