Exceedingly minute organisms of the simplest structure, being merely cells of varied forms, in the shape of spheres, rods, or intermediate shapes, which develop in infusions of organic matter, and multiply by fission with great rapidity, fraught, as happens, with life or death to the higher forms of being; conspicuous by the part they play in the process of fermentation and in the origin and progress of disease, and to the knowledge of which, and the purpose they serve in nature, so much has been contributed by the labours of M. Pasteur.