The bone marrow transplant is from a person with a gene that makes their white blood cells resistant to the virus. Once the person's old WBCs are gone (killed

The bone marrow transplant is from a person with a gene that makes their white blood cells resistant to the virus. Once the person's old WBCs are gone (killed by chemotherapy in the case of leukemia patients, which, IIRC, all these patients were due to the danger inherent in bone marrow transplants), the new bone marrow then produces only resistant cells.