From the book Chicken Soup for the Soul:
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.