In the same way, various forms of bacteria have the power of breaking down complex bodies in their immediate neighborhood, the products depending upon the substance, the kind of bacteria, and the conditions under which they act. Thus the bacteria lactis act upon the milk sugar present in milk, and convert it into lactic acid, thus bringing about the souring of milk.

Fig. 169.—Examples of Pathogenic Bacteria. (Drawn from photographs.)

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.