depends on his treatment regimen. Cancer treatments takes a toll on the patient's strength big time. It brings down your defence system, making you susceptible to infections which with leukemia your body's blood forming tissues already have a problem & cannot make mature WBCs (white blood count). Sometimes it's better off you stay away from people so you don't pick up a virus or a bacteria that may lead to another battle with life threatening infections. It takes time for the body, both red and white blood cells to be replenished. He might be getting blood transfusion too as is the case with most leukemias. Worst case scenario a bone marrow transfusion if the tx warrants that. It is in the bone marrow that new red blood cells is formed.