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Reasons for use
A wet nurse may be employed if the mother of a baby is unable to breast-feed her infant for reasons such as:
- drug use (prescription or illegal)
- insufficient production of breast milk (though see below).
Wet nurses have also been required following multiple births where the mother feels incapable of adequately nursing all of the children herself.
A woman can only serve as a wet nurse when she is lactating. It is often thought that this means the wet nurse must have recently given birth to a child of her own. This may be the case, but not necessarily, since regular suckling on a woman's breast can elicit the production of milk by a neural reflex.(E. Goljan, Pathology, 2nd ed. Mosby Elsevier, Rapid Review Series.)
The practice of using wet nurses is ancient and found in many cultures. Sometimes it is linked to social class. Members of property-owning classes had their children wet-nursed, in the hope of becoming pregnant again quickly to ensure an heir. (Lactation can suppress ovulation). Poor women, especially those who suffered the stigma of giving birth to an illegitimate child, sometimes had to give their baby up, temporarily or permanently, and a wet nurse would look after it.
Mohammed, the founder of Islam, was wet-nursed by a woman named Halimah bint Abdullah. In the culture of Arabia at that time, children who were nursed by the same woman, i.e. who grew up together as youngsters, were known as milk-siblings.
Napoleon was wet-nursed when he was a child by a woman called Camilla.
In Ancient China, the Emperor was often fed by a wet nurse as late as ten years old. 
Wet nurses were common for children of all social ranks in the southern United States during the 19th and early 20th centuries.
Wet nursing has sometimes been used with old or sick people who have trouble taking other nutrition. John Jacob Astor and John D. Rockefeller reportedly hired wet nurses for their own use in their old age. 
Through the widespread availability of infant formula, wet nurses are no longer necessary in developed nations and, therefore, are not common. Another substitute is expressed milk (or especially colostrum) donated to milk banks, analogous to blood banks. The use of a wet nurse is still a common practice in many developing countries.
Examples in fiction
- In William Shakespeare's Romeo and Juliet the character Nurse is Juliet's wet nurse. "Were not I thine only nurse, I would say thou hadst sucked wisdom from thy teat." 1.3.72
- In George Moore's novel Esther Waters, the eponymous heroine works as a wet nurse after the birth of her son while leaving him in the hands of a baby farmer.
- In John Steinbeck's novel The Grapes of Wrath, set in a time of great poverty, a woman whose baby has just died, and consequently whose breasts are engorged with milk, wet-nurses a man at the point of death, as no other nourishment is available.
- In the movie Spartacus, Crassus captures Spartacus's wife and baby. Since he wants Varinia as a concubine, he purchases a wet nurse for her baby. Varinia rejects his offer, saying, "I sent her away: I prefer to nurse the child myself."
- In Ayn Rand's novel Atlas Shrugged, the fresh-out-of-college government agent sent to spy on Hank Rearden's accounting is commonly referred to as "the wet nurse."