Nurse is the flavor name for a Backup Doctor. Once a Doctor has died, the Nurse will become an ordinary Doctor. The Nurse may or may not be self-aware (if not, it will receive a Vanilla Townie Role PM).
This role makes no guarantee about the existence of a Doctor in the setup to begin with. If there is no Doctor, then this role is functionally a Named Townie at best. On the other hand, in a game with multiple Doctors, the Nurse will generally inherit the sanity or other quirks of the Doctor who died first.
This role is pro-Town most of the time.
The self-aware version of Nurse (receives a Nurse or Backup Doctor Role PM), and only this self-aware version of Nurse, is considered Normal on mafiascum.net.
An older variant of Nurse also specifies that the Nurse's protection has a lower chance of succeeding. Some variations on that run with that concept alone instead of requiring a Doctor to die before the Nurse can protect.
Use and Power
A Nurse is used to ensure that the Doctor role stays in the game even if the first person holding the role dies. To that end, it is vaguely similar in power to giving a Doctor one shot of death immunity. However, considering that Doctor is used to prop up other, more powerful power roles, adding a Nurse simply helps perpetuate potential breaking strategies that may arise. Thus, Nurse is seldom used outside games specifically designed for it.