HOUSTON -- Derek Fields, a 6-month-old boy being treated for a rare combination of disorders diagnosed only once before, died yesterday in his mother's arms, his doctors said.

Derek was diagnosed last Sunday as suffering from Severe Combined Immunodeficiency, an inability to fight infection that was aggravated by a blood transfusion given him in mid-December for anemia.

Hospital officials said Derek began suffering complications from the rare combination of disorders late Friday and died shortly after midnight.

"The blood transfusion that would not normally affect another person caused graft-versus-host disease, where the white blood cells in that blood were attacking the baby," Dr. William Shearer, chief of allergies and immunology at Texas Children's Hospital, said.

Derek was only the second child with Severe Combined Immunodeficiency to be diagnosed as contracting graft-versus-host disease through a blood transfusion, Shearer said. The other child was successfully treated in Boston but later died of an infection, he said.

Derek suffered from the same rare disorder as David, the so-called "bubble boy" from Houston who lived in a sterile bubble for 12 years until his death two years ago. David, who did not have graft-versus-host disease, died following a bone marrow transplant that represented his only chance of leading a normal life.