Chemotherapy and/or radiation can be valuable treatments for some diseases, such as cancer. But along with the damage they can do to the bone marrow, these treatments can also destroy the immune system cells. And that might cause the body to reject the transplanted bone marrow. It may take 2 to 3 weeks or longer for the immune system and bone marrow to begin working again. During this time, the person is at high risk for infections. Isolation and infection control measures are used to protect the person from infection. The person may be in the hospital for 4 to 6 weeks, or longer.