The first change was infinitesimal—a mutation in a single nucleotide of a single cell, deep within her right breast. It is impossible to say what caused it, or even if it was necessarily destined to have any impact. Still, from that moment on, that cell was unique among the several trillion in her body. She was seventeen years old.

Over the next decade, the cell mutated several times. It began to behave autonomously, independent of the controls governing its neighbors. Its shape became slightly irregular. The structure of its nucleus changed. Its metabolism increased.

Her career was going exceedingly well.

Eight years later, the cell undergoes a sudden, dramatic change. The DNA within its unstable nucleus begins mutating hourly. All the cell’s energy is channeled into growth and reproduction. Normal signals directing it to stop are ignored. Immunological surveillance breaks down. Within a month, it has spawned close to a hundred daughter cells.

Sometimes she thinks she is living a fantasy. Between her two young children, her work, and her husband, she jokes that she doesn’t have time for problems.

It takes four years for the next great mutation. The malignant cells now number in the hundreds of thousands; but even if taken all together, are no larger than the head of a pin. Some, however, have already learned how to live outside the breast.

One afternoon, doing laps in the White House pool, she feels a dull pain in her lower back. She ignores it; seemingly just a minor muscle spasm. The backache lasts twenty-four hours and disappears as suddenly as it arrived.