When Emily Whitehead was six, she became the first child to receive genetically engineered T cells, an experimental treatment for leukemia. It cured her and helped launch her cell medicine field, as “Sunday Morning” contributor Kelefa Sane told Emily and her parents, Kari and Tom Whitehead, that she talk about how they beat cancer. And with Dr. Siddhartha Mukherjee, a leading oncologist and author of the new book, The Song of the Cell: An Exploration of Medicine and the New Human.