Death of Bacteria.—­Under certain circumstances, it would appear that the accumulation of the toxic products of bacterial action tends to interfere with the continued life and growth of the organisms themselves, and in this way the natural cure of certain diseases is brought about.  Outside the body, bacteria may be killed by starvation, by want of moisture, by being subjected to high temperature, or by the action of certain chemical agents of which carbolic acid, the perchloride and biniodide of mercury, and various chlorine preparations are the most powerful.

#Immunity.#—­Some persons are insusceptible to infection by certain diseases, from which they are said to enjoy a natural immunity.  In many acute diseases one attack protects the patient, for a time at least, from a second attack—­acquired immunity.

Phagocytosis.—­In the production of immunity the leucocytes and certain other cells play an important part in virtue of the power they possess of ingesting bacteria and of destroying them by a process of intra-cellular digestion.  To this process Metchnikoff gave the name of phagocytosis, and he recognised two forms of phagocytes:  (1) the microphages, which are the polymorpho-nuclear leucocytes of the blood; and (2) the macrophages, which include the larger hyaline leucocytes, endothelial cells, and connective-tissue corpuscles.

During the process of phagocytosis, the polymorpho-nuclear leucocytes in the circulating blood increase greatly in numbers (leucocytosis), as well as in their phagocytic action, and in the course of destroying the bacteria they produce certain ferments which enter the blood serum.  These are known as opsonins or alexins, and they act on the bacteria by a process comparable to narcotisation, and render them an easy prey for the phagocytes.

Artificial or Passive Immunity.—­A form of immunity can be induced by the introduction of protective substances obtained from an animal which has been actively immunised.  The process by which passive immunity is acquired depends upon the fact that as a result of the reaction between the specific virus of a particular disease (the antigen) and the tissues of the animal attacked, certain substances—­antibodies—­are produced, which when transferred to the body of a susceptible animal protect it against that disease.  The most important of these antibodies are the antitoxins.  From the study of the processes by which immunity is secured against the effects of bacterial action the serum and vaccine methods of treating certain infective diseases have been evolved.  The serum treatment is designed to furnish the patient with a sufficiency of antibodies to neutralise the infection.  The anti-diphtheritic and the anti-tetanic act by neutralising the specific toxins of the disease—­antitoxic serums; the anti-streptcoccic and the serum for anthrax act upon the bacteria—­anti-bacterial serums.