ut no infection results.

In most infections there is a focus where the infectious organisms are
localized; this may correspond to the point of entrance on a surface
or it may be in the interior of the body, the organisms being
deposited there after entrance. At this primary localization, the
_atrium_ of infection,[1] the organisms multiply and from this
point further invasion takes place. Many secondary foci may be formed
in the organs by distribution of the organisms, or there may be
infection of the blood and fluids of the body. The injuries which are
produced depend upon the nature of the infecting organisms. The most
common lesion consists in the death of the tissue about the infecting
organisms. In most cases the sum of the changes are so characteristic
that from them the nature of the infection is easily determined, and
these changes often give names to the disease; thus tuberculosis is a
disease characterized by the formation of tubercles or little nodules
in the body. The situation of the foci of disease is determined by
many conditions, the most important being the varying resistance of
the different organs of the body to the growth of bacteria. Certain
organs, such as the central nervous system, the muscles, the testicles
and the ovaries, have a high resistance to the growth of bacteria. The
disease may be localized in certain organs because only in these do
the bacteria find favorable conditions for growth. In spite of a high
general resistance to infection the lesions in chronic glanders are
most marked in the muscles, those of poliomyelitis in the spinal cord.
There are few bacterial diseases which are localized in the blood, but
many of the diseases caused by protozoa have this localization. In
every infection some organisms enter the blood, which acts as a
carrier and deposits them in the organs.

Bacteria cause disease by producing substances called toxines which
are poisonous to the cells, and of which two sorts are distinguished.
One form of toxines is produced by the bacteria as a sort of
secretion, and is formed both in the body and when the bacteria are
growing in cultures. Substances of this character, many of them highly
poisonous, are produced both by animals and plants. They may serve the
purpose both of offence and defence, as in the case of the snake
venom, and in other cases they seem to benefit their producers in no
way whatever, and may even be injurious to them. After the different
cere