It naturally occurs that to meet the various conditions under which decomposing matter exists in nature there is a great variety of species of bacteria, each species being adapted to a special field of operations. These species are distinguished from one another by the shapes they assume during their growth, some being rod shaped (the bacillus), others spherical (the coccus), and others spiral (the spirillum). Under one of these divisions the various species are classified. In these latter days of popular knowledge of scientific progress, but without precise information of details, bacteria are associated in the public mind with disease, especially of the epidemic form. While this prejudice is useful in stimulating the people to adopt and enforce preventive measures against conditions that tend to promote bacterial life in their homes and in their own persons, yet it should be understood that comparatively few of the great number and variety of bacteria are pathogenic, or disease producing, in man. So throughout the animal kingdom we find that few are susceptible to a common disease; or, in other words, that the same species of bacteria attack in equal force several varieties of animals. The explanation of this peculiarity is found in the variations of the quality or intimate nature of the tissues and organs of different species of animals. The same may be said of our own bodies--the several organs vary greatly in their susceptibility to the attacks of the different kinds of bacteria; hence the latter are classified as specific and nonspecific, according as they cause specific or nonspecific disease. The distribution of bacteria is limited only by the existence of plants and animals; that is, the existence of decomposing vegetable and animal matter. Though they are more abundant in the earth where such matter is found most abundantly, yet they abound in the air, the water, on plants, animals, and insects, on our own bodies, and in every cavity leading to the exterior. As bacteria are always searching for food, the number present is a sure indication of the degree of cleanliness of the thing, individual, or locality where they are found. The movements of bacteria from one point to another are through the medium of some other mode of conveyance than their own bodies afford. Thus they are borne by the water, by vegetation, by animals of every kind, especially insects, by the air on particles of dust. The typhoid bacillus, borne in water and milk, has caused innumerable epidemics of that dreaded disease. [Sidenote: The Deadly Tubercle Bacillus]