Zuby Malik is an unlikely candidate to violate international law. A 78-year-old mother of four with a crown of silver hair, she is a retired obstetrician-gynecologist with a penchant for order.

But Ms. Malik is fighting for her life. After receiving a Stage 4 non-small-cell lung cancer diagnosis a year ago, she exhausted many of the treatments available to her and grappled with torturous side effects that left her itching and gasping for breath. During the summer, she decided to go to Cuba and bring back a cancer vaccine that is not approved in the United States. That she comes from a family steeped in medical training made the decision all the more difficult.

“At first, I was a little nervous,” said Ms. Malik, sitting in her Northern California living room flanked by an oxygen tank and a table of medicines. “But American treatments were not helping me, and I decided I should go to Cuba. What other choice did I have?”

Soon after she began the medication, she said, her breathing became easier and her energy returned. In her refrigerator was a box of blue- and orange-capped vials of the vaccine.