Many years ago, when I worked as a volunteer at Stanford
Hospital, I got  to know a little girl named Liza who was suffering from a rare and
 serious disease.  Her only chance of recovery appeared to be a blood transfusion from
 her  five-year-old brother, who had miraculously survived the same
disease  and had developed the antibodies needed to
 combat the illness.  The doctor explained the situation to her
  little brother, and asked the boy if he would be willing to give his
  blood to his sister.

  I saw him hesitate for only a moment before taking a deep breath and
  saying, "Yes, I'll do it if it will save Liza."
  As the transfusion progressed, he lay in bed next to his sister
  and smiled, as we all did,  seeing the color returning to her
cheeks.  Then his face grew pale and his smile faded.
  He looked up at the doctor and asked with a trembling voice,
  "Will I start to die right away?"
  Being young, the boy had misunderstood the doctor; he thought he was
  going to have to give her all his blood.