His attention was next naturally directed to the diseases of animals and human beings. His studies in fermentations and in silkworm diseases had taught him the use of the microscope for such investigations. Splenic fever--known also as anthrax--a disease that attacks most species of domestic animals and may also prove fatal to man, was the first to yield the secret of its origin. The cause proved to be a bacterium--that is, a small, rod-shaped plant. This was but the first of a series of similar discoveries, until now the science of bacteriology has become one of the most important branches of knowledge. Pasteur's investigations included much more, however, than the mere discovery of the germ of the disease. He showed that a series of diseases which passed under different names in different animals were all due to the same cause. Further, he discovered one of the methods of distributing the disease. When the carcasses of animals that have died from the disease are not buried deeply below the surface of the ground, animals grazing above may become infected with the disease. {309} The germs of the disease can be shown to occur in the grass above the graves. It is carried to the surface in the bodies of earth-worms. This important observation was the first hint of the methods of disease distribution by some living intermediary. Modern medicine has come to understand that these biological distributing agents are far more important than the fabled transmission through the air. Pasteur overturned the notion of spontaneous generation of life. Then his work eradicated the idea of the spontaneous generation of disease. It opened up a new era by showing that the origin of many diseases is not due to changes in the atmosphere nor to some morbid productivity of soil or water under favoring circumstances, but to minute living organisms whose multiplication is encouraged by the conditions that were supposed to produce disease. Finally, came the precious suggestion that living things always convey and distribute disease; man to man, for epidemics travel not with the velocity of the wind but only as fast as the means of communication between distant points; animal to man, as is well known, for many diseases now; and, lastly, insects, worms and the like were also shown to be real carriers of disease.