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.