I'd always imagined wet nurses to be some sort of matronly, bosomy, super-lactating women, merrily feeding aristocratic babies for years and years while their own children were at school.
I didn't realise they were largely from workhouses and had either lost their own babies or were so desperate that they abandoned their own, usually illegitimate children or left them in the charge of the workhouse, and many of these children then died. 
Interesting article about it here