Leukemia is cancer that starts in blood-forming cells

Leukemia is a type of cancer that begins in the bone marrow—the soft tissue inside bones where blood cells are made. Instead of producing healthy blood cells in the right amounts, the bone marrow starts making abnormal white blood cells that multiply out of control. These cancer cells crowd out normal blood cells, which means your body cannot make enough healthy red cells, white cells, or platelets to work the way it should.

The word "leukemia" comes from the Greek words for "white" and "blood," because the disease involves an overproduction of white blood cells. But the problem is not just that there are too many of them—it is that they do not work properly and interfere with the job of normal blood cells.

Key Takeaways

  • Leukemia starts in bone marrow when blood-forming cells begin to divide abnormally and multiply without stopping.
  • The four main types are acute lymphoblastic leukemia (ALL), acute myeloid leukemia (AML), chronic lymphocytic leukemia (CLL), and chronic myeloid leukemia (CML), divided by how fast they grow and which cells are affected.
  • Symptoms can include fatigue, frequent infections, unusual bruising or bleeding, and swollen lymph nodes, though some people have no symptoms early on.
  • Diagnosis requires blood tests and usually a bone marrow biopsy, which takes a small sample of marrow to examine under a microscope.
  • Treatment depends on the type, stage, and your age, and may include chemotherapy, targeted therapy, radiation, or stem cell transplant.

The four main types of leukemia

Leukemia is divided into types based on two things: how fast the cancer grows, and which kind of blood cell it starts in. Acute leukemias grow quickly and need treatment right away. Chronic leukemias grow slowly and may not cause symptoms for years, though they still need monitoring and treatment.

Acute lymphoblastic leukemia (ALL) starts in lymphoid cells (a type of white blood cell) and grows fast. It is the most common leukemia in children, though it can happen at any age. Acute myeloid leukemia (AML) starts in myeloid cells and also grows quickly. It is more common in adults but can occur in children.

Chronic lymphocytic leukemia (CLL) develops slowly in lymphoid cells and is usually found in people over 55. Many people live for years without needing treatment. Chronic myeloid leukemia (CML) starts in myeloid cells and grows slowly. It is often discovered by accident during a routine blood test.

Why leukemia develops

Leukemia happens when a mutation (a change in the DNA) occurs in a blood-forming cell in the bone marrow. This mutation causes the cell to divide over and over without stopping, and the cancer cells do not die the way normal cells do. Scientists do not fully understand why these mutations happen in most cases, but they know some factors increase the risk.

Risk factors include exposure to high levels of radiation, certain chemicals like benzene, some inherited genetic conditions, and previous cancer treatment. Age matters too—some types are more common in children, others in older adults. Having a family history of leukemia raises the risk slightly, though most people with leukemia have no family history of the disease.

Importantly, leukemia is not contagious. You cannot catch it from another person, and nothing you did or did not do caused it to develop.

How leukemia affects the body

As leukemia cells multiply, they take up space in the bone marrow where healthy blood cells should be made. This means your body produces fewer red blood cells, normal white blood cells, and platelets. Each type of blood cell has a job: red cells carry oxygen, white cells fight infection, and platelets help blood clot.

When you do not have enough red blood cells, you feel tired and short of breath—this is anemia. Without enough normal white blood cells, you get infections more easily and they last longer. Low platelets mean you bruise easily, bleed longer from small cuts, or have nosebleeds and bleeding gums. Leukemia cells can also spread to the lymph nodes, spleen, liver, brain, or other organs, causing swelling or other problems.

Symptoms and when they appear

Symptoms of acute leukemia often come on suddenly over days or weeks. They include fatigue that does not go away with rest, fever, frequent infections, unusual bruising or bleeding (including nosebleeds, bleeding gums, or blood in urine or stool), and swollen lymph nodes, spleen, or liver. Some people have bone or joint pain.

Chronic leukemia may cause no symptoms at all, especially early on. It is often found by accident when a blood test is done for another reason. When symptoms do appear, they are usually the same as acute leukemia but develop more slowly. Because chronic leukemia grows slowly, some people can live for years without treatment, though they need regular blood tests to monitor the disease.

Having these symptoms does not mean you have leukemia—many other conditions cause fatigue, fever, or swollen lymph nodes. But if symptoms last more than a few weeks, a doctor needs to investigate.

How leukemia is diagnosed

Diagnosis starts with a blood test. A complete blood count (CBC) shows how many of each type of blood cell you have and whether the numbers are abnormal. If the CBC is abnormal, the doctor will order more blood tests to look at the cells under a microscope and identify what type of leukemia it might be.

The next step is usually a bone marrow biopsy. The doctor uses a needle to take a small sample of bone marrow, usually from the hip bone. This sample is examined under a microscope to see what the cells look like and confirm the diagnosis. The sample may also be tested for specific genetic changes that help determine which type of leukemia it is and guide treatment choices.

Other tests may include imaging scans (like a chest X-ray or CT scan) to see if leukemia cells have spread to other organs, and lumbar puncture (spinal tap) to check if cancer cells are in the fluid around the brain and spinal cord.

Treatment depends on the type and stage

Treatment for leukemia varies widely depending on which type you have, how advanced it is, your age, and your overall health. Chemotherapy uses drugs to kill cancer cells and is the main treatment for most leukemias. It may be given by mouth, injection, or through an IV, and treatment usually happens in cycles over weeks or months.

Targeted therapy uses drugs that attack specific genetic changes in leukemia cells. For example, CML is often treated with tyrosine kinase inhibitors, drugs that target a specific mutation found in most CML cases. Radiation therapy uses high-energy beams to kill cancer cells and is sometimes used to treat leukemia in the brain or before a stem cell transplant.

Stem cell transplant (also called bone marrow transplant) is used for some people, especially those with high-risk disease or who relapse after initial treatment. Healthy blood-forming cells from a donor replace the diseased bone marrow. Other newer treatments, like CAR-T cell therapy, teach immune cells to recognize and attack leukemia cells.

For some chronic leukemias that are not causing symptoms, doctors may recommend "watch and wait"—regular blood tests and monitoring without immediate treatment, since starting treatment too early does not improve outcomes.

Frequently Asked Questions

Is leukemia always fatal?

No. Survival depends on the type of leukemia, how advanced it is at diagnosis, age, and how well the person responds to treatment. Some types, especially ALL in children, have high survival rates. Others, like AML, have lower rates but many people still go into remission and live for years. Chronic leukemias often allow people to live a normal lifespan with treatment.

Can you get leukemia from someone else?

No, leukemia is not contagious. You cannot catch it from a family member, friend, or anyone else. It develops from mutations in a person's own cells and cannot be passed through contact, blood, or any other means.

What is remission?

Remission means that after treatment, blood tests and bone marrow biopsies show no signs of leukemia cells. Complete remission means no leukemia can be detected. Remission does not always mean the leukemia is cured—some people relapse and need more treatment—but it means the cancer is under control.

Does leukemia run in families?

Most leukemia is not inherited. Having a family member with leukemia raises your risk slightly, but most people with leukemia have no family history of the disease. Some rare inherited conditions do increase leukemia risk, but these are uncommon.

How long does leukemia treatment take?

Treatment length varies. Acute leukemia treatment may take several months to a year or more. Chronic leukemia may require ongoing treatment for years or even a lifetime, depending on the type and how well it responds. Some people take daily pills; others have treatment cycles with breaks in between.