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.

STRUCTURE AND DEVELOPMENT OF PARASITES

The group of parasites consists of two general classes, the vegetable, and the animal. In the former class belong the bacteria, and in the latter the protozoa. The two classes are not sharply differentiated, but in general the vegetable parasites are less highly organized than the animal.

BACTERIA