Zoe Gilbert, 11, has something called bone marrow failure, which doctors believe can only be a cured by a transplant.
The condition, a form of aplastic anaemia, affects the blood and is caused by the bone marrow and stem cells not producing enough blood cells.
So far, Zoe's desperate family have failed to find a match for the youngster. Instead, the schoolgirl is having her dangerously weak immune system boosted - by horse blood.
A serum from the equine blood is injected into Zoe because it contains antithymocyte globulin (ATG) - an antibody which doctors hope will kick-start her immune system.
The treatment has a range of strange side effects, including the growth of facial hair.
Zoe will have to wait for four weeks after the treatment is completed to find out whether it has been successful.
If it doesn't work, her only option will be finding a suitable bone marrow donor to come forward and save her life.
Zoe's mother Elizabeth Gilbert has made an urgent appeal for more donors to sign up to the bone marrow register to 'secure Zoe's life'.
She said: "It's very difficult at the moment not knowing what will happen. My biggest fear is it not working and having to go through it again.
"A transplant would secure Zoe's life. Even if we don't find a match,donors could save other people's lives."
Zoe, who lives with her family in Impington, Cambridgeshire, was diagnosed with the condition by doctors at Addenbrooke's Hospital after she had 10 nose bleeds in just two weeks.
The condition means her bone marrow fails to reproduce red and white blood cells and platelets, which prevent bruising, and leaves Zoe with little resistance to infections.
Children who do not have a brother or sister who is a good bone marrow match can be treated with a immunosuppressive therapy using antibodies from horse and rabbit blood.
The horse therapy reduces the number of white blood cells circulating in the bloodstream using drugs such as antilymphocyte globulin (ATG) and ciclosporin, which stimulate the bone marrow to restart blood cell production.
To help Zoe visit facebook.com/thehelpzoecampaign
Anthony Nolan runs the UK register of bone marrow donors. Donors need to be between the ages of 16-30 and in good health. For more information, visit www.anthonynolan.org