Milk fever, known to fancy types as parturient paresis, affects highly productive dairy cattle, whose calcium metabolism is seriously challenged by the volume of milk that they have been bred to produce. When a "downer" cow cannot resorb enough mineral calcium from her bones for use in her milk, she experiences blood hypocalcemia. Because calcium in an essential ion for the transmission of muscular, and some neural, electrical signals, this leads to general loss of muscle activity, including weakness and inability to stand.

Source: Structure and Function of Domestic Animals, by Dr. Bruce Currie, and AnSci 150 by Dr. Ron Butler.