The basic cause: DNA damage in blood-forming cells

Leukemia starts when something damages the DNA inside cells that make blood. These cells live in your bone marrow — the soft tissue inside your bones — and normally follow strict rules: divide when needed, stop when told, and die on schedule. When DNA gets damaged, a cell can break those rules. It divides over and over without stopping, and the copies don't die the way they should. Eventually these broken cells crowd out the healthy ones, and you have leukemia.

The damage itself is not inherited in most cases. It happens by chance in a single cell, usually during that person's lifetime. The cell then passes the damaged DNA to its copies, creating a clone of broken cells. This is why leukemia is not contagious and why most people with leukemia have no family history of it.

Key Takeaways

  • Leukemia begins when DNA damage causes a blood cell to divide uncontrollably instead of following normal rules.
  • Most leukemia is not inherited; the DNA damage happens by chance in a single cell during a person's lifetime.
  • Radiation, certain chemicals, and some viruses can increase the risk of DNA damage, but most people exposed to these do not develop leukemia.
  • Age, genetics, and previous cancer treatment are risk factors that doctors know about, though they cannot predict who will develop leukemia.
  • In children, leukemia usually has no clear cause that doctors can identify.

Radiation exposure and leukemia risk

High doses of radiation can damage DNA in bone marrow cells. This includes radiation from nuclear accidents, atomic weapons testing, and medical radiation like repeated CT scans or radiation therapy for another cancer. People who survived the atomic bombs dropped on Japan in 1945 had higher rates of leukemia in the years afterward. Workers in nuclear plants and people treated with radiation for other cancers also show increased risk.

The risk depends on the dose and the age when exposure happened. Children exposed to radiation are at higher risk than adults. However, the radiation from a single X-ray or CT scan is not considered high enough to cause leukemia in most people. The risk becomes measurable only at doses much higher than what a typical medical scan delivers.

Chemical exposure as a risk factor

Certain chemicals can damage DNA and raise the risk of leukemia. Benzene — used in manufacturing and found in gasoline — is the best-documented example. Workers in shoe factories, oil refineries, and chemical plants who breathed benzene over many years developed leukemia at higher rates than the general population. Formaldehyde, used in building materials and some industrial processes, has also been linked to increased risk.

Like radiation, chemical risk depends on how much exposure happened and for how long. A single exposure to a small amount is unlikely to cause leukemia. The risk rises with repeated or long-term contact. Most people who work with these chemicals do not develop leukemia, which shows that exposure alone is not enough — other factors also play a role.

Viruses and leukemia

A few viruses can increase leukemia risk, though this is rare in developed countries. Human T-cell leukemia virus type 1 (HTLV-1) causes a specific type of leukemia in some infected people, mainly in parts of Japan, the Caribbean, and Africa. Human immunodeficiency virus (HIV) raises the risk of certain blood cancers, including some leukemias, because it weakens the immune system.

Most people infected with these viruses do not develop leukemia. The virus alone is not enough; other factors must be present. These viruses are not spread through casual contact like a cold or flu.

Age and inherited genetic conditions

Age is one of the strongest risk factors for leukemia. Acute myeloid leukemia (AML) and chronic lymphocytic leukemia (CLL) are far more common in people over 65. Acute lymphoblastic leukemia (ALL) is most common in children under five, then becomes rare until adulthood.

Some inherited genetic conditions raise leukemia risk. People with Down syndrome have a higher chance of developing leukemia in childhood. Fanconi anemia, Bloom syndrome, and neurofibromatosis type 1 are rare inherited conditions that also increase risk. Families with a history of leukemia may carry genetic changes that raise risk, though most family members will never develop the disease.

Previous cancer treatment

Chemotherapy and radiation used to treat another cancer can damage bone marrow cells and lead to leukemia years later. This is called therapy-related leukemia or secondary leukemia. It is a known side effect of cancer treatment, but it happens in a small percentage of people treated. The risk is higher with certain chemotherapy drugs and with combined chemotherapy and radiation.

Doctors weigh this risk against the benefit of treating the original cancer. For most people, the benefit of treating their first cancer outweighs the small risk of leukemia later. Doctors monitor people who have had cancer treatment, watching for signs of leukemia through blood tests.

Why most leukemia has no clear cause

In the majority of leukemia cases — especially in children — doctors cannot point to a specific cause. The person was not exposed to known radiation or chemicals, does not carry an inherited genetic condition, and has no family history. The DNA damage simply happened by chance, the way random mutations occur in cells throughout life.

This randomness is frustrating for patients and families who want to know why it happened. It also means that leukemia is not preventable in most cases. You cannot reduce your risk by avoiding something, because the cause is not something you did or were exposed to. Research continues to identify new risk factors and to understand why some cells develop damage while others do not.

Frequently Asked Questions

Is leukemia hereditary?

Most leukemia is not hereditary. The DNA damage happens by chance in a single cell during a person's lifetime and is not passed down from parents. However, some rare inherited genetic conditions do raise leukemia risk. If you have a family history of leukemia, talk to your doctor about whether genetic testing might be useful.

Can you catch leukemia from someone else?

No. Leukemia is not contagious. You cannot catch it from another person through contact, blood, or any other means. The only exception is that a pregnant person can pass certain viruses (like HTLV-1) to a child, which may raise the child's risk, but this is rare in developed countries.

Does stress cause leukemia?

There is no scientific evidence that stress causes leukemia. Stress can affect the immune system and overall health, but it does not cause the DNA damage that starts leukemia. Leukemia develops from specific damage to blood-forming cells, not from emotional or physical stress.

Can diet or lifestyle prevent leukemia?

There is no diet or lifestyle change that prevents leukemia, because most leukemia results from random DNA damage rather than something you did or did not do. Staying healthy through exercise and good nutrition supports overall health, but it does not reduce leukemia risk.

If my parent had leukemia, will I get it?

Most likely not. Having a parent with leukemia does not mean you will develop it, because most leukemia is not inherited. Your risk is slightly higher than the general population, but it remains low. Talk to your doctor if you are concerned about your family history.