The presence of peculiar forms of these bacteria in diseases has so suggested the relation of cause and effect as to have given rise to the so-called “germ theory” of disease. Now we know with reasonable certainty that cholera, diphtheria, typhoid fever, whooping cough, mumps, cerebro-spinal meningitis, pneumonia, tuberculosis, hydrophobia, and many other diseases have each its specific cause in the form of a microbe. [Illustration: FIG. 176. BACILLUS OF TUBERCULOSIS IN SPUTUM. BACILLUS OF DIPHTHERIA (KLEBS-LOEFFLER). BACILLUS OF TYPHOID FEVER. (Photo-Micrographs, 1,000 diam., by William M. Gray, M. D.)] [Illustration: TERTIAN FORM. AESTIVO-AUTUMNAL FORM. FIG. 177.--BLOOD OF MAN. SHOWING PARASITE OF MALARIA (LAVERAN). (Photo-Micrographs, 1,000 diam., by William M. Gray, M. D.)] Henle, a German physiologist, as early as 1840, maintained the doctrine of _contagium vivum_, or contagion by the transmission of living germs. Certain classes of diseases have also long been known as zymotic, or ferment diseases. Louis Pasteur’s work, however, marks the first definite and important results in the study of bacteriology, and he is the father of the “germ theory” of disease. He exploded the previously held theories of scientists concerning the spontaneous generation of living things, and clearly established and promulgated the knowledge of disease germs. Commencing his great work about 1865 with the investigation of the silk worm plague in France, he discovered it to be due to parasites, and checked it. He also gave great attention to the subject of fermentation, proving it to be caused by micro-organisms. Taking up the diseases of men and animals, he gave practical value to the truths of his theory in the treatment of hydrophobia, diphtheria, and other diseases, using the principle of vaccination to destroy or render innocuous the toxins or disease-producing poisons derived from living germs. Working along the same lines must be mentioned Dr. Koch, whose success in detecting the microbes which cause consumption and cholera has made him famous the world over. Of the great variety of these little microbes which have been separately identified, many are innocuous, and, in fact, subserve many important and useful purposes in nature, while others are to be as much dreaded as the deadly cobra or the rattlesnake. A few typical examples of the latter are given in Figs. 176 and 177, multiplied 1,000 diameters. The illustrations represented in Fig. 177 show the parasites that cause malaria, or fever and ague. The dark bean-shaped cells are the normal blood corpuscles, and the few speckled cells are those infested with the malarial parasites. It is now believed that the mosquito is the active factor in the dissemination of malaria, and it is, therefore, to be remembered that this pestiferous little insect not only inflicts a painful and disagreeable sensation with his puncture, but innoculates the system with poisonous malarial germs at the same time.