That the disease-producing, or pathogenic bacteria, are able to infect milk supplies is shown by the fact that numerous epidemics of contagious disease have been directly traced to milk infection. Milk is generally consumed in a raw state, and as a considerable number of this class of organisms are able not only to live but actually grow in milk, which is such an ideal culture-medium for the development of most bacteria, it is not surprising that disease processes should be traced to this source. The organisms in milk capable of causing disease do not alter or change its physical properties sufficiently to enable their presence to be detected by a physical examination. =Origin of pathogenic bacteria in milk.= Disease-producing bacteria may be grouped, with reference to their relation toward milk, into two classes, depending upon the manner in which infection occurs: Class I. Disease-producing bacteria capable of being transmitted directly from a diseased animal to man through the medium of infected milk. Class II. Bacteria pathogenic for man but not for cattle, which are capable of thriving in milk after it is drawn from the animal. In the first group, the disease produced by the specific organism must be common to both cattle and man. The organism must live a parasitic life in the animal, developing in the udder, and so infect the udder. It may, of course, happen that diseases toward which domestic animals alone are susceptible may be spread from one animal to another in this way without affecting human beings. In the second group the bacterial species live a saprophytic existence, growing in milk, as in any other nutrient medium, if it happens to find its way therein. In such cases, milk indirectly serves as an agent in the dissemination of disease, by giving conditions favorable to the growth of the disease germ. By far the most important of diseases that may be transmitted directly from animal to man through a milk supply is tuberculosis, but in addition to this, foot and mouth disease (aphthous fever in children), Malta fever, and acute enteric troubles have also been traced to a similar source of infection. The most important specific diseases that are disseminated through subsequent infection of the milk are typhoid fever, diphtheria, scarlet fever, and cholera, but, of course, the possibility exists that any disease germ capable of living and thriving in milk may be spread in this way. In addition to these diseases that are caused by the introduction of specific organisms (the causal organism of scarlet fever has not yet been definitely determined), there are a large number of more or less illy defined troubles of an intestinal character that occur especially in infants and young children that are undoubtedly attributable to the activity of micro-organisms that gain access to milk during and subsequent to the milking, and which produce changes in milk before or after its ingestion that result in the formation of toxic products.