- "That's the Florence Nightingale effect. It happens in hospitals when nurses fall in love with their patients."
- —Doc explaining to Marty why his mother fell in love with him in 1955.
The Florence Nightingale effect, named for a famous trainer of nurses during the Crimean War, describes when a doctor or nurse becomes infatuated with their patient in a hospital.
Lorraine described how she fell in love with George, who looked like "a little lost puppy", while taking care of him after her father hit him with his car. In Timeline 2, Marty was hit instead and took the place of George in Lorraine's bedroom, and therefore, she fell in love with Marty for the same reason.
Behind the scenes
Although there may have been caregivers that have fallen in love with their patients, the editors of Wikipedia were not able to find the phrase "Florence Nightingale effect" in literature before Back to the Future used the phrase, and it has rarely been used elsewhere.  This suggests that the term may been invented specifically for this film in order to explain Lorraine's motivation.