The nurse was a Mondasian and a descendant of the original skeleton crew of a Mondasian colony ship caught in the gravitational pull of a black hole. Because of the black hole's time dilation, the bottom of the ship had a much faster flow of time than the top of the ship.
The nurse was the surgeon's assistant. At some point they established Operation Exodus. It involved augmenting hospital patients by turning them into cybernetic humanoids so that they would survive the journey to the top of the ship. However, the process was extremely painful and to keep its nature a secret, the nurse would typically silence any patients who cried out in pain by turning the volume on their voice synthesiser down.
The nurse was present when Bill was tricked by Razor into entering the conversion theatre, taunting her about the process as the surgeon explained the device which inhibits emotions was the handlebars of the headset. As such, Bill was transformed into the first fully successful Mondasian Cyberman. She presumably suffered the same fate when the Cybermen increased their rate of upgrading to pursue the Doctor. (TV: World Enough and Time)