Blood cancers are cancers in which different types of blood cells start to grow out of control. They can start in the bone marrow, where new blood cells are made, or in the lymphatic system, which helps the body fight infections. There are many kinds of blood cancer, including different types of leukemia and lymphoma, as well as multiple myeloma. Some grow quickly, while others develop more slowly. Some types of blood cancer affect both children and adults, especially certain types of leukemia and lymphoma.