exist among the species in Nature as well as in artificial cultures.  The bacteria which produce the various wound infections and abscesses, etc., appear to vary under normal conditions from a type capable of producing violent and fatal blood poisoning to a type producing only a simple abscess, or even to a type that is entirely innocuous.  It is this factor, doubtless, which in a large measure determines the severity of any epidemic of a bacterial contagious disease.

Susceptibility of the individual.

The very great modification of our early views has affected our ideas as to the power which individuals have of resisting the invasion of pathogenic bacteria.  It has from the first been understood that some individuals are more susceptible to disease than others, and in attempting to determine the significance of this fact many valuable and interesting discoveries have been made.  After the exposure to the disease there follows a period of some length in which there are no discernible effects.  This is followed by the onset of the disease and its development to a crisis, and, if this be passed, by a recovery.  The general course of a germ disease is divided into three stages:  the stage of incubation, the development of the disease, and the recovery.  The susceptibility of the body to a disease may be best considered under the three heads of Invasion, Resistance, Recovery.

Means of Invasion.—­In order that a germ disease should arise in an individual, it is first necessary that the special bacterium which causes the disease should get into the body.  There are several channels through which bacteria can thus find entrance; these are through the mouth, through the nose, through the skin, and occasionally through excretory ducts.  Those which come through the mouth come with the food or drink which we swallow; those which enter through the nose must be traced to the air; and those which enter through the skin come in most cases through contact with some infected object, such as direct contact with the body of an infected person or his clothing or some objects he has handled, etc.  Occasionally, perhaps, the bacteria may get into the skin from the air, but this is certainly uncommon and confined to a few diseases.  There are here two facts of the utmost importance for every one to understand:  first, that the chance of disease bacteria being carried to us through the air is very slight and confined to a few diseases, such as smallpox, tuberculosis, scarlet fever; etc., and, secondly, that the uninjured skin and the uninjured mucous membrane also is almost a sure protection against the invasion of the bacteria.  If the skin is whole, without bruises or cuts, bacteria can seldom, if ever, find passage through it.  These two facts are of the utmost importance, since of all sources of infection we have the least power to guard against infection through the air, and since of all means ’of