Under favorable conditions of temperature, such as prevail in the warm months of summer and early autumn, micro-organisms grow with almost incomprehensible rapidity in any substance which is suitable food for them. Milk is such a substance; and, as bacteria multiply with wonderful rapidity, millions forming in a few hours in every thimbleful,[56] it is perfectly evident that they must produce something. This something may or may not be of a harmful nature, depending upon what species of organism produces it. I have no evidence at hand to show what is the nature of the product of any one organism which finds a home in milk; but there are instances on record where the nature of the product of certain bacteria is known: for example, the diphtheria bacillus. This little rod, growing upon the outside of the tonsils in the human throat, produces a most virulent poison, which, taken up by the circulation, pervades the whole body, and often so enfeebles its functions as to destroy it.[57] Reasoning from analogy, it is not impossible to suppose that other organisms may produce substances of a similar character, poisonous in their effects, and which, when taken into the alimentary canal, may produce very grave digestive disorders.[58] Further, bacteria, by their multiplication, use some of the constituents of milk for their food, thus changing its composition. It is very important to prevent this growth, or, in case it has begun, to check it before it has rendered the milk unwholesome food. Hence the necessity of sterilizing _immediately_ all milk which is not received directly from the cow. Besides, cows are often infected with tuberculosis, foot-and-mouth disease, splenic fever, pneumonia, and other dangerous disorders. Their milk may be a direct cause of infection. When it is sterilized there is less danger from it; but even then it is not, of course, a wholesome food, because of the poisons which may be produced in the animal during the progress of the disease, and because a sick and weakened cow cannot give wholesome milk.[59] In many cities, through the influence of children's hospitals and sanitariums, the knowledge and methods of sterilizing milk for infants' food are gradually spreading. Circular wire frames, made something like casters, and fitted with eight bottles, each holding enough milk for one feeding, may be bought for the purpose of sterilizing at almost any pharmacy. The frame is to be set in a kettle with water in the bottom, which on boiling produces steam, the heat of which does the sterilizing.[60] This is an easy method. Another good way is to sterilize at a lower temperature for a longer time, as less change is produced in the constituents of the milk by the lower degree of heat. This may be easily done by immersing the bottles in water at 190° Fahr., and maintaining that temperature for an hour.[61]