What causes leukemia(blood cancer) and how to prevent it?

Leukemia: In theory, it is possible that some leukemias are caused by unknown viruses. Very high dose radiation raises the risk (e.g. people living in Hiroshima or Nagasaki in the years after the atom bombs). But almost all cases have no known cause and no known way to prevent them. Fortunately, leukemia is rare. If you're generally healthy, please don't be worried about it.

Depends: Most of the time leukemia, at least in young people, is just bad luck. Rarely leukemia is caused by a genetic problem. In either case, there isn't much you can do to prevent the disease. In adults, there are several rarer forms of leukemia that can be caused by a retrovirus similar to HIV called HTLV. This virus can be prevented by the use of condoms with sexual contact.