Bacteria are minute fungi, each consisting of a single cell enclosed in a cell membrane of cellulose which can be demonstrated by iodine, the latter causing the protoplasm to retract from the cell wall. There is no nucleus or central core. Some of the bacteria are colorless, others pigmented, yellow, blue or red. The cells vary in shape and in size in different species as well as in their mode of growth, and are named in accordance with these peculiarities. The round or oval cells are called _cocci_; the rod-shaped organisms are termed _bacilli_. The cocci are called _micrococci_ or _macrococci_ according to their size; _diplococci_ or _tetracocci_, according to the production of pairs or groups of four in their multiplication; _streptococci_, because in their growth they always form chains of cells; _staphylococci_, because they grow in irregular clusters resembling bunches of grapes. Some of the bacteria have the power of motion generally produced by cilia or flagella and others are motionless. +Habitat.+ These organisms may truly be said to be omnipresent. Every thing we wear or use or eat, even the air itself, is impregnated with them. Pathogenic germs may also be found among these myriads. Every species has its own particular habitat, where the conditions especially favor its growth, just as any of the larger plants require a certain soil, a supply of water, temperature, and proper amount of light in order to make growth and multiplication possible. The bacteria in the air are more numerous in dry weather, being carried up as dust by the wind, for a moist surface holds any bacteria which may lie upon it. So complete is the action of moisture, that air, which contained 600 microorganisms when inspired, has been shown to return from the lungs with almost none, the moist respiratory surfaces catching and holding the bacteria; so that the expired air is practically sterilized; this is true even when the expiration is from diseased lungs. The act of coughing, however, may expel bacteria in the mucus ejected. The number of bacteria in the air is very variable, but is much greater in houses than out of doors, and is naturally increased by attempts to clean the rooms. +Parasitic Nature.+ The number of species of pathogenic germs is comparatively small compared with the number of all the varieties of germs, for the latter are practically innumerable. Indeed, the wonderful qualities of resistance in animal tissues is the only thing that makes animal life possible and it is this power of resistance that allows certain wounds to heal by primary union when left without protection or care.