For the first six months of his life, Zachary Hazan had to live in a sealed bubble to protect himself from infections.

Zachary was born with Severe Combined Immunodeficiency Disease, or bubble-boy disease, a rare genetic condition that disables the immune system's infection- fighting antibodies and T cells.

"He had this chest infection that was really taking over his body, and that's the reason he wasn't growing," said his father, Erik. "All his strength was going to fighting this thing. He was really on the edge."

Zachary (front) and Jacob Hazan.

Then a transplant of his brother's stem cells 18 months ago helped free the two-year-old from a quarantine where he had to use a telephone to speak to his family.

Zachary likely wouldn't have lived a decade ago. But his case has given hope to doctors attending an immunology conference in Montreal this week looking into new practices and treatments.

• INDEPTH: Stem cells

Dr. Fritz Melchers

In Zachary's case, doctors took some bone marrow stem cells from his older brother, Jacob, and implanted them in the infant.

Immunologists aren't sure why the procedure worked.

"We wish we knew, because it is against all rules of immunology," said Dr. Fritz Melchers, head of the International Union of Immunological Societies.

Melchers said researchers are experimenting with repairing a patient's own genes and implanting them back into the body to rescue damaged immune systems.

"It's very exciting," said Fred Modell, who lost a son to an immune deficiency almost 20 years ago. He and his wife, Vicki, established the Jeffrey Modell Foundation, in memory of their son.

"There are so many protocols which are appropriate that Vicki and I often think maybe if Jeffrey was [born] at a different time, they may have been able to save him," said Modell, who now raises money to fight the disease.