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 SHAPE.--Bacteria are composed of single cells and are consequently called unicellular organisms. Under the microscope individual cells are seen to differ in size, shape, and structure. In shape bacteria show three different types; the rod-shaped (bacillus), the spherical (coccus), and the spiral (spirillum). The organisms causing typhoid fever for example are a variety of bacilli, those causing pneumonia are cocci, while those causing Asiatic cholera are spirilla. [Illustration: FIG. 1.--BACILLI OF VARIOUS FORMS. (_Williams._)] SIZE.--Bacteria vary greatly in size. Average rod-shaped bacteria are about 1/25000 of an inch long, but there are undoubtedly organisms so small that they cannot be seen, even by means of the strongest microscopes we now possess. [Illustration: STAPHYLOCOCCI. STREPTOCOCCI. DIPLOCOCCI. TETRADS. SARCINÆ. FIG. 2.--(_Williams._)] MOTION.--The power of motion in certain species of bacteria is due to hair-like appendages called flagella. These flagella by a lashing movement somewhat resembling the action of oars enable the organisms to move through fluids. MULTIPLICATION.--After bacteria have fully developed, each cell divides into two equal parts; the process of division is called fission. Each of these two parts rapidly grows into a full-sized organism. Then fission again takes place, so that four bacteria replace the original one. In each of the four, fission occurs again, and so the process of multiplication continues. As bacteria develop they group themselves in characteristic ways. Some, like the streptococci, arrange themselves in chains; the diplococci, in pairs; the tetrads, in groups of four; others in packets called sarcinæ, and still others, the staphylococci, form masses supposed to resemble bunches of grapes.