About 30,000 people in the United States have cystic fibrosis, a life-threatening disease that often leads to lung damage. The severity of the disease varies, and new research has found one reason why some patients do worse than others.

Stacy Danko was diagnosed with cystic fibrosis when she was a child. Now in her 40s, she wonders why her sister, Chrissy, died from the disease at age 12, yet her own disease never seemed as severe.

“I think every day, 'Why? Why am I so fortunate?'” Danko said.

Researchers collected data from the U.S. Cystic Fibrosis Twin and Sibling Study. They compared exposure to second-hand smoke in the home with a gene abnormality that some patients carry.

“What we really wanted to know is what makes one patient or helps one patient do better than another,” said Dr. Garry Cutting with Johns Hopkins University.

Out of about 800 patients studied, those exposed to second-hand smoke lost, on average, 10 percent of their lung function compared with those not exposed. When those patients also carried a specific gene abnormality, their loss of lung function doubled to about 20 percent.

That might explain why the disease is more severe in some patients than others.

“For the first time, we now see that other genes play a role in how bad that effect is on the lungs of cystic fibrosis patients,” Cutting said.

The study appeared in the Journal of the American Medical Association. Danko and her sister were both exposed to second-hand smoke as children, and her boyfriend smokes. She'd like to know if she carries the gene.

“If you know you carry the gene that is going to make it affect you more, then you're going to be that much more careful,” Danko said. “You're going to be more open with individuals smoking around you. You'll just avoid it.”

Danko said she still hopes for a cure and tries to protect the 40 percent lung function she has left.