to be developed as long as the bacteria continue to grow, whether in a milk pail or a human stomach.  If now the poisons are absorbed by the body, they may produce a mild or severe disease which will be more or less lasting, continuing perhaps as long as the same food and the same bacteria are supplied to the individual.  The most important disease of this class appears to be the dreaded cholera infantum, so common among infants who feed upon cow’s milk in warm weather.  It is easy to understand the nature of this disease when we remember the great number of bacteria in milk, especially in hot weather, and when we remember that the delicate organism of the infant will be thrown at once into disorder by slight amounts of poison which would have no appreciable effect upon the stronger adult.  We can easily understand, further, how the disease readily yields to treatment if care is taken to sterilize the milk given to the patient.

We do not know to-day the extent of the troubles which are produced by bacteria of this sort.  They will, of course, be chiefly connected with our food products, and commonly, though not always, will affect the digestive functions.  It is probable that many of the cases of summer diarrhoea are produced by some such cause, and if they could be traced to their source would be found to be produced by bacterial poisons swallowed with food or drink, or by similar poisons produced by bacteria growing in such food after it is swallowed by the individual.  In hot weather, when bacteria are so abundant everywhere and growing so rapidly, it is impossible to avoid such dangers completely without exercising over all food a guard which would be decidedly oppressive.  It is well to bear in mind, however, that the most common and most dangerous source of such poisons is milk or its products, and for this reason one should hesitate to drink milk in hot weather unless it is either quite fresh or has been boiled to destroy its bacteria.

Pathogenic bacteria which are true parasites.

This class of pathogenic bacteria includes those which actually invade the body and feed upon its tissues instead of living simply upon swallowed food.  It is difficult, however, to draw any sharp line separating the two classes.  The bacteria which cause diphtheria (Fig. 28), for instance, do not really invade the body.  They grow in the throat, attached to its walls, and are confined to this external location or to the superficial tissues.  This bacillus is, in short, only found in the mouth and throat, and is practically confined to the so-called false membranes.  It never enters any of the tissues of the body, although attached to the mucous membrane.  It grows vigorously in this membrane, and there secretes or in some way produces extremely violent poisons.  These poisons are then absorbed by the body and give rise to the general symptoms of the disease.  Much the same is true of the bacillus which causes tetanus or lockjaw (Fig.