Stories for the heart

I read this story in the book 'Stories for the Heart' and it always  makes me tear up.

'There is a true story of a little boy whose sister needed a
blood transfusion. The doctor explained that she had the same
disease the boy had recovered from two years earlier. Her only
chance of recovery was a transfusion from someone who had previously
conquered the disease. Since the two children had the same rare blood type, the
boy was an ideal donor. "Would you give your blood to Mary?" the doctor asked.
Johnny hesitated. His lower lip started to tremble. Then he smiled and said "Sure, for my sister."
Soon the children where wheeled into the hospital room. Mary, pale and thin, Johnny, robust and healthy.
Neither spoke, but when their eyes met, Johnny grinned. As the nurse inserted the needle into his arm,
Johnny's smile faded. He watched the blood flow through the tube. With the ordeal almost over, Johnny's voice,
slightly shaky, broke the silence.

"Doctor, when do I die?"


Only then did the the doctor realize why Johnny had hesitated, why his lip had trembled when
he agreed to donate his blood. He thought giving his blood to his sister would mean giving  up his life.
In that brief moment, he had made his great decision.'