In 2012, Sandy Jauregui-Baza was hiking along the Tamul waterfall in San Luis Potosí, Mexico, when she started coughing and having trouble breathing. "I remember thinking I must be coming down with something," she recalls.

Jauregui-Baza was an avid exerciser; she ran or hiked daily, logging more than 100 miles each month. She ate clean, avoiding almost all processed foods. She figured she was too healthy for anything to be seriously wrong. But after developing flu-like symptoms, she went to an urgent care clinic in Los Angeles. The doctor thought it might be tuberculosis, based on the results of her cloudy chest X-ray and her recent honeymoon in Nepal, where the infectious disease is common. But a few days later, when the definitive test for TB came back negative, doctors did a lung biopsy to look for other causes.

The final diagnosis: Jauregui-Baza had stage IV lung cancer, the most advanced form of the disease; it had spread into the bones of her spinal column. "I thought the doctors had to be kidding," says Jauregui-Baza, now 32. "I've never even smoked, and I had just hiked to the base camp of Mount Everest. How could I have lung cancer?" The prognosis was grim: More than 95 percent of stage IV lung cancer patients succumb to the disease within five years of diagnosis. Jauregui-Baza was given just six months to live.