This is most frequent in the case of milk, where it has been found that whole districts supplied by one milk farm have been affected with some disease, such as typhoid fever, diphtheria, or scarlet fever, and inquiries have shown that either at the farm or in the milk shop germs of these diseases have got into the milk, either from the air, from sewer gas, or more often from water taken from an impure source, and either added to the milk as an adulteration, or used for washing out the milk cans. These diseases carried by milk, as well as tuberculosis from the milk of tuberculous cows, can be entirely prevented by boiling the milk for at least five minutes before it is used.