Now, while most species of bacteria are harmless, some are the cause of sickness and death when they gain admittance to the body under certain conditions.  These disease-producing bacteria (known as pathogenic), when established in the blood and tissues of the body, bring about important chemical changes, depending upon the species of bacteria, and also produce a particular form of disease.  The production of certain diseases by the agency of bacteria has now been proved beyond all doubt.  In yellow fever, erysipelas, diphtheria, typhoid fever, consumption and other diseases, the connection has been definitely established.

The evil results these germs of disease produce vary greatly in kind and severity.  Thus the bacteria of Asiatic cholera and diphtheria may destroy life in a few hours, while those of consumption may take years to produce a fatal result.  Again, the bacteria may attack some particular organ, or group of organs, and produce mostly local symptoms.  Thus in a boil there is painful swelling due to the local effect of the bacteria, with slight general disturbance.

398.  The Battle against Bacteria.  When we reflect upon the terrible ravages made by infectious diseases, and all their attendant evils for these many years, we can the better appreciate the work done of late years by tireless scientists in their efforts to modify the activity of disease-producing bacteria.  It is now possible to cultivate certain pathogenic bacteria, and by modifying the conditions under which they are grown, to destroy their violence.

In brief, science has taught us, within certain limitations, how to change the virulent germs of a few diseases into harmless microbes.

399.  Alcoholic Fermentation and Bacteria.  Men of the lowest, as well as of the highest, type of civilization have always known that when the sugary juice of any fruit is left to itself for a time, at a moderately warm temperature, a change takes place under certain conditions, and the result is a liquid which, when drank, produces a pronounced effect upon the body.  In brief, man has long known how to make for himself alcoholic beverages, by means of which he may become intoxicated with their poisonous ingredients.

Whether it is a degraded South Sea Islander making a crude intoxicant from a sugary plant, a Japanese preparing his favorite alcoholic beverage from the fermentation of rice by means of a fungus plant grown for the purpose, a farmer of this country making cider from fermenting apple juice, or a French expert manufacturing costly champagne by a complicated process, the outcome and the intent are one and the same.  The essential thing is to produce an alcoholic beverage which will have a marked physiological effect.  This effect is poisonous, and is due solely to the alcoholic ingredient, without which man would have little or no use for the otherwise harmless liquid.

While the practical process of making some form of alcoholic beverage has been understood for these many centuries, the real reason of this remarkable change in a wholesome fruit juice was not known until revealed by recent progress in chemistry, and by the use of the microscope.  We know now that the change is due to fermentation, brought about from the influence, and by the action, of bacteria (sec. 125).