When Someone Gets a Bone Marrow Transplant, They Have to Be

When someone gets a bone marrow transplant, they have to be quarantined and carefully protected from pathogens for a period of time afterward. Why?

A) It's a traumatic procedure, and any extra stress on their body could kill them.
B) Bone marrow produces blood cells, so they have insufficient circulation for a period of time so their immune system lacks the energy to fight off any infection.
C) Anytime a foreign substance, even someone else's bone marrow, is introduced to a body, there is an extra risk of infection.
D) Bone marrow produces the main cells involved in the specific immune response, so these patients are at special risk of infection until they build up enough marrow to produce sufficient cells.
E) A common complication of bone marrow transplants is infection, and doctor's don't want the infection to spread to others.