To understand how infection affects the body involves an inquiry as to the nature of infection, its mode of entrance into the body, and its operation on its organs and tissues. The terms “infection” and “contagion” are often used as synonymous; but a strict definition according to the medical significance of each limits the former to “the transmission of disease by actual contact of the diseased part with a healthy absorbent or abraded surface,” and the latter to “transmission through the atmosphere by floating germs.” But in the final analysis the cause of disease in both infection and contagion is so similar in its action that the medical profession has adopted the term “communicable disease” in all cases where the disease is communicated from one person to another by means of a germ, whatever may be its method of attack on the body. The common characteristic of “communicable diseases” is their germ origin. [Sidenote: What the Germ Is] What is this communicable germ or agent? A bacterium--a little stick, staff--so called from the rodlike shape it assumes in the process of growth. The individual bacterium (plural, bacteria) is an organism representing a low form of vegetable life; resembles mold; in size the smallest living thing that can be seen with the microscope; in masses forming the films floating on foul fluids or covering decomposing animal or vegetable matter. It consists of a single cell, and its mode of increase when placed under proper conditions of growth is by division of the cell body; the two cells formed out of the first being divided into four before complete separation has taken place; the four dividing into eight, the eight into sixteen, the sixteen into thirty-two, and so on indefinitely. Now, as it requires only thirty minutes for one cell to divide, it has been estimated that a single bacterium will in twenty-four hours increase to the number of over sixteen million five hundred thousand, and in forty-eight hours to two hundred and eighty-one million five hundred thousand. At this rate of increase, in three days there would be a mass of bacteria weighing about sixteen million pounds. As the multiplication of bacteria depends upon conditions that soon interfere with or interrupt their growth, as the want of food, their own secretions, and certain natural forces operating against them, these stupendous figures are useful only as an illustration of the enormous fertility of these organisms, and their destructive energy when they attack a susceptible living body. [Sidenote: The Function of Bacteria]