What causes cancer?
Uncontrolled cell division
Sometimes when cells divide, there are mutations in the DNA, and in extremely rare cases there can be a mutation sequence that causes the cell to keep on dividing (and so will its copies). Cells do normally not do this. And because we have so many cells in the body, the chance of getting it is relatively large, but the body takes care of cancer in most cases before it's noticed.
When the cells keep on replicating and not dying, there will eventually be a problem.
Cancer can occur in any of the body's cells.
Leukemia is when you get cancer in the area that produces white blood cells. It causes your body to produce more than you need and it's hard to deal with. You then have to take bone marrow from the patient and wipe out all white blood cells, and later place back the bone marrow so that your body can produce new healthy blood cells.