At four months of age, A.J. Williams weighed just 8 pounds. "We could see his little chest cave in as he struggled to breathe," recalls Erika Williams, A.J.'s mother. Several doctors near their home in Atlanta, Ga., had told A.J.'s parents that his vomiting, diarrhea and weight loss were just "something that's going around." A.J. also had thrush, an infection in his mouth, that wouldn't go away.

Five days after yet another trip to the doctor's office, A.J.'s mom and dad knew that something was seriously wrong and that their tiny son needed big help.

"We took him to the emergency room at the local hospital, and as soon as the nurse saw him, she told me to quit filling out the paperwork and bring him to the back," Erika Williams remembers. "They told us that symptoms like A.J.'s were often caused by a bad valve in the heart."

An ultrasound revealed that A.J.'s heart wasn't the problem, but that he had a form of pneumonia commonly found in AIDS patients. Additional tests ruled out AIDS, but showed that A.J. suffered from severe combined immunodeficiency (SCID). Babies with SCID are born without a functioning immune system and so are susceptible to fungal, bacterial and viral infections. SCID is sometimes called "bubble boy disease" because of a well-known SCID child many years ago who was put into a special "sterile bubble" to protect him from infections. Left unidentified and untreated, SCID infants seldom survive beyond one year.

SCID is treated by transfusing bone marrow stem cells from a healthy tissue-matched donor. These transplanted stem cells mature in the recipient into T and B cells - the specialized white blood cells, called lymphocytes that form a critical part of the immune system in fighting infection. Because A.J. didn't have a matched donor, he needed a special kind of transplant in which the bone marrow stem cells from a parent are purified prior to the transplant. His mother was selected as the closest match.

Dr. Morton J. Cowan, a pediatric immunologist and bone marrow transplant specialist at UCSF Benioff Children's Hospital, is one of the few doctors in the country who perform this specialized procedure.

A donor arranged for the Angel Flight organization to fly the Williams family to California. On May 30, Cowan transplanted some of Erika Williams' stem cells into her son.

A.J. faced one more hurdle after the successful transplant. "He had gotten used to not sucking the bottle," explains Erika Williams, so A.J. had to use a feeding tube for over a year to help him gain much-needed weight.

"He's now 3 years old, and there's nothing he can't do," his mother says. "We're constantly trying to keep him out of mischief — all the regular little boy stuff. It's great!"