Leukemia is a type of cancer usually associated with children that affects cells in your blood. Most of the time, although not always, leukemia affects the white blood cells. The disease starts in the bone marrow before spreading to other parts of the body. Leukemia is generally described as either acute leukemia (fast-growing) or chronic leukemia (slower-growing). The latter type of leukemia is most often found in children.
The types of leukaemia are:
Acute Myelogenous (granulocytic) Leukemia (AML)
Chronic Myelogenous (granulocytic) Leukemia (CML)
Acute Lymphocytic (lymphoblastic) Leukemia (ALL)
Chronic Lymphocytic Leukemia (CLL)
Treatment: Chemotherapy and possibly stem cell transplant
We don't know what causes leukaemia, but research is going on all the time to try and find the answer.
Young people with certain rare genetic conditions, like Down's Syndrome or Li-Fraumeni Syndrome, are at an increased risk of getting Leukaemia compared to the general population. A genetic condition is something you are born with.
There are a few non-cancerous conditions, such as aplastic anaemia or myelodysplastic syndromes, that may also increase the risk of developing Leukaemia.
I am sorry about your relative. I hope that (s)he will be ok, and I hope this info helps you and your family.