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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
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