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The invention of the microscope, as we have seen, revealed the existence of innumerable little plants and animals, so small that even many millions crowded together are invisible to the naked eye. These tiny living creatures are called micro-organisms or germs. The plant forms are called bacteria (singular, bacterium), and the animal forms protozoa (singular, protozoön). The common belief that all or even most bacteria are harmful is quite unfounded. As a matter of fact, while not less than 1500 different kinds of micro-organisms or germs are known, only about 75 varieties are known to produce disease.

Most bacteria belong to the class of micro-organisms called saprophytes, which find their food in dead organic matter, both animal and vegetable, and cannot flourish in living tissues. These saprophytes act upon the tissues of dead animals and vegetables, and resolve them into simpler substances, which are then ready to serve as nourishment for plants higher in the vegetable kingdom. Thus the processes which we know as fermentation and putrefaction are due to the action of saprophytes. Higher plants in turn furnish food for men and animals, and so the food supply is used over and over in different forms, making what is known as the food cycle. If it were not for bacterial activities vegetation would be robbed of its supply of nourishment, and plant life would speedily end; destruction of plant life would deprive the animal kingdom of food and thus all life would become extinct. The saprophytes are consequently essential to the existence of both animals and vegetables.

There are, however, other organisms called parasites, which can exist in living tissues of animals or vegetables. The organisms at whose expense the parasites live are called their hosts. Parasites not only contribute nothing to their hosts, but generally harm them by producing poisonous substances or depriving them of food. Some parasites are able to lead a saprophytic existence also, but as a rule they live at the expense of animal or plant life. Pathogenic, or disease-producing, germs belong to the group of parasites. The pathogenic germs which find favorable soil in the body produce poisons called toxins. These poisons or toxins interfere with the bodily functions, and thus cause what we know as communicable disease. Communicable diseases are caused by specific germs only: that is, a certain disease cannot develop unless its particular germs are present; the germs of typhoid for instance, can cause typhoid fever only, and not tuberculosis or other disease.

A number of diseases are caused by micro-organisms that are now well known. Chief among these diseases are colds, septicæmia (blood poisoning), influenza, pneumonia, diphtheria, typhoid fever, tuberculosis, whooping cough, Asiatic cholera, bubonic plague, meningitis, tetanus ("lock jaw"), leprosy, gonorrhœa, syphilis, relapsing fever, typhus fever, glanders, and anthrax. Micro-organisms not yet identified probably cause the communicable diseases whose origin is not known with certainty. These include infantile paralysis, smallpox, scarlet fever, measles, mumps, chicken-pox, Rocky Mountain spotted fever, yellow fever, hydrophobia (rabies), foot-and-mouth disease. We can hardly doubt that the intensive laboratory research now in progress will reveal in the near future the specific germs of these diseases also.