CARRIERS OF DISEASE It has now been discovered that a great number of human diseases are caused by microscopic parasites, which are spoken of in a general way by the name invented by the great Pasteur, viz. "microbes." Wool-sorter's disease, Eastern relapsing fever, lock-jaw, glanders, leprosy, phthisis, diphtheria, cholera, Oriental plague, typhoid fever, Malta fever, septic poisoning and gangrene have been shown to be caused each by a peculiar species of the excessively minute parasitic vegetables known as bacteria (or Schizophyta). Others, for example, malaria and sleeping sickness, have been shown to be caused by almost equally minute microbes, which are of an animal nature, and similar to the free-living animalcules which we call Protozoa, or "simplest animals," whilst a third lot of diseases--rabies, smallpox, yellow fever, scarlet fever, and typhus--are held to be caused by similar minute parasites, although these have not yet actually been seen and cultivated, but are surely inferred (from the nature and spread of these diseases) to exist. The difference of the microbes called bacteria from the disease-causing microbes classed as "Protozoa" consists in their simpler structure and mode of growth. They are essentially filaments which continually multiply by fission--a process often carried so far that the little organisms present themselves as short rods, or as curved (comma-shaped), or even spherical particles (micrococci)--and only in favourable conditions arrest their self-division so as to grow for a time into the thread-like or filament shape. Often these filaments are not straight, but spirally twisted, and are called "spirilla." Some of them are blood parasites, but the larger number attack the tissues, and others occur in the digestive canal.