Leukemia is a group of blood cancers that usually begin in the bone marrow and produce high numbers of abnormal blood cells. These blood cells are not fully developed and are called blasts or leukemia cells.

Leukemia is cancer of the body's blood-forming tissues, including the bone marrow and the lymphatic system.

Many types of leukemia exist. Some forms of leukemia are more common in children. Other forms of leukemia occur mostly in adults. 

Leukemia usually involves the white blood cells. Your white blood cells are potent infection fighters — they normally grow and divide in an orderly way, as your body needs them. But in people with leukemia, the bone marrow produces an excessive amount of abnormal white blood cells, which don't function properly.

Leukemia is thought to occur when some blood cells acquire changes (mutations) in their genetic material or DNA. A cell's DNA contains the instructions that tell a cell what to do. Normally, the DNA tells the cell to grow at a set rate and to die at a set time. In leukemia, the mutations tell the blood cells to continue growing and dividing.

When this happens, blood cell production becomes out of control. Over time, these abnormal cells can crowd out healthy blood cells in the bone marrow, leading to fewer healthy white blood cells, red blood cells and platelets, causing the signs and symptoms of leukemia.

SYMPTOMS

• Fever or chills
• Persistent fatigue, weakness
• Frequent or severe infections
• Losing weight without trying
• Swollen lymph nodes, enlarged liver or spleen
• Easy bleeding or bruising
• Recurrent nosebleeds
• Tiny red spots in your skin (petechiae)
• Excessive sweating, especially at night
• Bone pain or tenderness

TYPES OF LEUKEMIA

• Acute lymphocytic leukemia (ALL). This is the most common type of leukemia in young children. ALL can also occur in adults.
• Acute myelogenous leukemia (AML). AML is a common type of leukemia. It occurs in children and adults. AML is the most common type of acute leukemia in adults.
• Chronic lymphocytic leukemia (CLL). With CLL, the most common chronic adult leukemia, you may feel well for years without needing treatment.
• Chronic myelogenous leukemia (CML). This type of leukemia mainly affects adults. A person with CML may have few or no symptoms for months or years before entering a phase in which the leukemia cells grow more quickly.
• Other types. Other, rarer types of leukemia exist, including hairy cell leukemia, myelodysplastic syndromes and myeloproliferative disorders.