In some of the infective conditions the conflict fortifies the organism against future attacks of the same nature, as for example in the immunity following many of the acute infective diseases.
The parasitic hypothesis postulates the invasion of a parasite from without, thus making a new growth an infective process.
Many cancer-parasites have been described in cancerous growths, including bacteria, yeasts and protozoa, but the innumerable attempts made to demonstrate the causal infective organism have all completely failed.
Hyaline degeneration is found in certain acute infective conditions; the toxins specially act on these connective-tissue cell elements.
By embolism is meant the more or less sudden stoppage of a vessel by a plug of solid matter carried thither by the current of the blood; be it a little clot from the heart or, what is far more pernicious, an infective fragment from some focus of infection in the body, by which messengers new foci of infection may be scattered about the body.
But whatever merits they had as clarifiers of turbid water, the advent of bacteriology, and the recognition of the fact that the bacteria of certain diseases may be water-borne, introduced a new criterion of effectiveness, and it was perceived that the removal of solid particles, or even of organic impurities (which were realized to be important not so much because they are dangerous to health per se as because their presence affords grounds for suspecting that the water in which they occur has been exposed to circumstances permitting contamination with infective disease), was not sufficient; the filter must also prevent the passage of pathogenic organisms, and so render the water sterile bacteriologically.
Of late years enormous impulse has been given to our knowledge of the causation of disease by microbes, through the works of Gaspard, who injected putrid matter into the veins of a living animal; by Villemin, who discovered that tuberculosis is infective; by Davaine; and especially by Pasteur, Koch and others too numerous to mention, who have worked, and are still working, at the microbic causation of disease with marvellous success.
Mitchill (1764-1831) pointed out in America the resemblance which exists between symptoms of Anti- poisoning by snake venom and infective fevers.'
Weir Mitchell and others have shown that serpent venom consists chiefly of albumoses, and the toxins formed by infective bacilli have a somewhat similar chemical nature.
The same method of serum therapeutics has been used in other infective diseases, but not with the same success.
The virulence of infective diseases varies in different epidemics, and at different times in the same epidemic. It had been noted that many infective diseases did not attack an individual a second time, the first attack appearing to protect from subsequent ones.
Although the anti-toxins which are used in the cure of infective diseases are not dangerous to life, yet they sometimes cause unpleasant consequences, more especially an urticarial eruption almost exactly like that which follows eating mussels or other shell-fish.
The demonstration by Pasteur that definite diseases could be produced by bacteria, proved a great stimulus to research in the etiology of infective conditions, and the result Historical was a rapid advance in human knowledge.
The key to the artificial establishment of active immunity is given by the fact long established that recovery from an attack of certain infective diseases is accompanied by protection for varying periods of time against a subsequent attack.
This, as already pointed out, depends upon the increase of opsonins, though it is also to be noted that in many infective conditions there is another factor present, namely a leucocytosis, that is, an increase of the leucocytes in the blood, and the defensive powers of the body are thereby increased.
In the case of the latter animal the serum infective conditions led Metchnikoff to place great p g importance on phagocytosis.
Abbott, Principles of Bacteriology (7th ed., London, 1905); Crookshank, Bacteriology and Infective Diseases (with bibliography, 4th ed., London, 1896); Duclaux, Traite de microbiologie (Paris, 1899-1900); Eyre, Bacteriological Technique (Philadelphia and London, 1902); Flugge, Die Mikroorganismen (3rd ed., Leipzig, 1896); Fischer, Vorlesungen fiber Bakterien (2nd ed., Jena, 1902); Gunther, Einfiihrung in das Studium der Bakteriologie (6th ed., Leipzig, 1906); Hewlett, Manual of Bacteriology (2nd ed., London, 1902); Hueppe, Principles of Bacteriology (translation, London, 1899); Klein, Micro-organisms and Disease (3rd ed., London, 1896); Kolle and Wassermann, Handbuch der pathogenen Mikroorganismen (Jena, 1904) (supplements are still being published; this is the most important work on the subject); Lofler, Vorlesungen fiber die geschichtliche Entwickelung der Lehre von der Bacterien (Leipzig, 1887); M`Farland, Text-book upon the Pathogenic Bacteria (5th ed., London, 1906); Muir and Ritchie, Manual of Bacteriology (with bibliography, 4th ed., Edin.
The bacteriology of the infective diseases (with bibliography) is fully given in the System of Medicine, edited by Clifford Allbutt, (2nd ed., London, 1907).
The most important works on immunity are: Ehrlich, Studies in Immunity (English translation, New York, 5906), and Metchnikoff, Immunity in Infective Diseases (English translation, Cambridge, 1905).