TUBA CITY, Ariz. — Lorria Trujillo never felt she knew enough to question doctors about her 6-month-old daughter's health. She didn't second-guess them when they insisted Charlotte merely had a viral infection after months of being sick, nor question them when the girl's lungs collapsed.

Trujillo watched as her baby was unhooked from life support and held her until she died.

That was 1995.

Written on the child's death certificate is "severe infection," but Trujillo now knows the condition that claimed her daughter's life is the same one her 9-year-old daughter, Grace Marie Yazzie, suffers from.

"With most families, it's the mother that's really responsible for taking care of their babies," the Navajo woman said. "I really felt like I didn't do my part as a mother, and I kept looking for something I would have missed. Would I have known?"

Without treatment, children have no chance of surviving severe combined immune deficiency — a disorder that's more commonly referred to as "bubble boy disease" after a Houston boy who was forced to spend his 12-year life in a plastic bubble free of germs.

In the Navajo population, one in every 2,500 children inherits SCID, a condition that gives them virtually no immune system. In the general population, SCID is much more rare, affecting one in 100,000 children.

Before Grace was born, Trujillo had researched SCID, and knowing that she and her now ex-husband had a one in four chance of having another baby with the condition, she insisted on a blood test. Although doctors didn't think the test was necessary, Trujillo knew not to keep quiet this time around.

"The longer it took, the more apprehensive I got," Trujillo said. "When I saw the doctor come into the delivery room, I could tell there was something wrong. He told me, 'Yeah, she tested positive for SCID."'

Prior to the late 1970s, the illness baffled doctors working with Navajo children. Over generations, families would lose children without explanation.

Dr. Mortan Cowan, who has worked with SCID patients for more than two decades, encountered his first case in the mid-1980s when he was asked to watch over Navajo patients for a doctor in Denver who went on sabbatical.

To Cowan, the disease appeared to be linked to genes, so in 1986 he and a research geneticist decided over lunch to find the gene — a quest that would take 15 years, Cowan said.

"When we ultimately found the gene and went back, we were able to show that it was the same gene mutation in every Navajo and Apache child that had the disease," Cowan said.

American Indians typically have had a higher infant mortality rate than other ethnic groups because of poverty and limited access to medical care, said Dr. Diana Hu, chief pediatrician on the 27,000 square-mile Navajo Nation. So when an infant died of infection, "you don't really notice that is odd," because others also were dying of infection, she said.