refaction 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, septicaemia (blood
poisoning), influenza, pneumonia, diphtheria, typhoid fever,
tuberculosis, whooping cough, Asiatic cholera, bubonic plague,
meningitis, tetanus ("lock jaw"), leprosy, gonorrhoea, 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.

STRUCTURE AND DEVELOPMENT OF PARASITES

The group of parasites consists of two general classes, the vegetable,