The milk of diseased animals should never be used for food.  There is no way by which such milk can invariably be detected, but Prof.  Vaughan, of Michigan University, notes the following kinds of milk to be avoided: 

1.  Milk which becomes sour and curdles within a few hours after it has been drawn, and before any cream forms on its surface.  This is known in some sections as ‘curdly’ milk, and it comes from cows with certain inflammatory affections of the udder, or digestive diseases, or those which have been overdriven or worried.

2.  “Bitter-sweet milk” has cream of a bitter taste, is covered with ‘blisters,’ and frequently with a fine mold.  Butter and cheese made from such milk cannot be eaten on account of the disagreeable taste.

3.  ‘Slimy milk’ can be drawn out into fine, ropy fibers.  It has an unpleasant taste, which is most marked in the cream.  The causes which lead to the secretion of this milk are not known.

4.  ‘Blue milk’ is characterized by the appearance on its surface, eighteen or twenty-four hours after it is drawn, of small, indigo-blue spots, which rapidly enlarge until the whole surface is covered with a blue film.  If the milk be allowed to stand a few days, the blue is converted into a greenish or reddish color.  This coloration of the milk is due to the growth of microscopic organisms.  The butter made from ‘blue milk’ is dirty-white, gelatinous, and bitter.

5.  ‘Barnyard milk’ is a term used to designate milk taken from unclean animals, or those which have been kept in filthy, unventilated stables.  The milk absorbs and carries the odors, which are often plainly perceptible.  Such milk may not be poisonous, but it is repulsive.

There is no doubt that milk often serves as the vehicle for the distribution of the germs of various contagious diseases, like scarlet fever, diphtheria, and typhoid fever, from becoming contaminated in some way, either from the hands of milkers or from water used as an adulterant or in cleansing the milk vessels.  Recent investigations have also shown that cows are to some extent subject to scarlet fever, the same as human beings, and that milk from infected cows will produce the same disease in the consumer.

Milk should not be kept in brass or copper vessels or in earthen-ware lined with lead glazing; for if the milk becomes acid, it is likely to unite with the metal and form a poisonous compound.  Glass and granite ware are better materials in which to keep milk.

Milk should never be allowed to stand uncovered in an occupied room, especially a sitting-room or bedroom, as its dust is likely to contain disease-germs, which falling into the milk, may become a source of serious illness to the consumer.  Indeed it is safest to keep milk covered whenever set away, to exclude the germs which are at all times present in the air.  A good way is to protect the dishes containing milk with several layers of cheese-cloth, which will permit the air but not the germs to circulate in and out of the pans.  Neither should it be allowed to stand where there are strong odors, as it readily takes up by absorption any odors to which it is exposed.