Explainer: What is cancer?

It’s a word no one wants to hear for themselves or someone they love: cancer. It’s so scary that some people instead call it “the C-word.” There are more than 200 types of cancer. No two are exactly alike. Some still baffle doctors and scientists while others can be treated and even cured.

Our cells are supposed to grow and divide. Without more cells, our bodies couldn’t develop, and some parts would wear out.

Cells divide as we shed skin and grow hair. Dividing cells keep our stomach lining working and replenish our blood. Cells grow to make our muscles bigger and bones longer.

Cells usually keep this growth and division under tight control. But if those controls fail, a cell can begin to divide rapidly. And the new cells can do the same. This out-of-control division and growth is called cancer. Doctors call cancers that spread to other parts of the body metastases (Meh-TASS-tuh-sees).

What causes cancer?

Normally, genes control whether a cell grows or divides into two. Genes are bits of DNA that act as instruction manuals for our cells. Every time a cell divides, its DNA gets copied. This makes sure both new cells have the same instructions.

Just as printed instructions can end up with typos, a gene’s DNA can pick up “spelling errors” as it is copied. Scientists call these changes mutations.

To make sure such errors get corrected, the cell has DNA repair genes. These produce molecules that run along the DNA, repairing mutations. Most times, a cell repairs the spelling errors. Other times, the body’s immune system may find this cell and kill it. And some genetic changes are so severe that they won’t let the cell survive.

But once in a while, a cell with mutations sneaks through.

Some of those mutations may be harmless. Others might change how a cell works, but not turn it cancerous. But mutations in any of these three types of genes can increase the odds a cell will turn cancerous.

DNA repair genes. Without DNA repair, more mutations can slip through.

Proto-oncogenes. These genes control cell growth. A mutation here can cause a cell to grow when it’s not supposed to. Once a gene has such a mutation, it’s called an oncogene. Onco comes from the Greek oncos for mass or bulk (like the mass of a tumor).

Tumor-suppressor genes. These genes usually work to control proto-oncogenes; they stop them from making cells grow too much. A mutated tumor-suppressor gene might not do this.

Some mutations are inherited — passed down from our parents. Damage caused by the environment can lead to others. For instance, exposure to the sun’s ultraviolet rays can cause DNA mutations. So can inhaling the smoke from cigarettes. Factors that trigger mutations raise the risk of developing cancer. Such agents are called carcinogens.

Sometimes, scientists and doctors can point to an inherited gene or to an environmental cause for a cancer. Many times, however, they will never find a cancer’s cause.

Cancer may sound scary. Here’s how it forms — and how your body works to keep things in check. Fortunately, most of the time your body is able to stop it before it starts.

Types of cancer

Cancer can start from many types of mutations. It also can occur in different types of cells and anywhere in the body. This means that no two cancers are the same. Scientists tend to define cancers based on the body part in which they first emerged.

Some cancers form a solid mass known as a tumor. Not all tumors are cancers, however. Some are just lumps, where cells divided too much but did not spread. If doctors catch a cancerous tumor before its cells start spreading to other parts of the body, they can remove it. Common solid cancers include breast cancer, skin cancer and lung cancer.

Other cancers affect the blood. These can form in bone marrow, which produces blood cells. They also can form in immune organs and lymph (a fluid that fights infection and carries away waste). These cancers attack cells that move through the blood, such as red or white blood cells. Blood cancers include leukemia and lymphoma.

Although cancers are most common in adults, children can get them, too. In 2024, some 9,600 U.S. kids under age 14 were diagnosed with cancer. More than four in every five kids at these ages with cancer will survive. Sadly, however, some 1,000 children in the United States will die from cancer each year.

For kids under age 14, the most common types of cancer are leukemia and lymphoma. Less common types include cancers in the brain, spinal cord and other nerve tissues.

Data: NCCR*Explorer: National Cancer Institute

This graph shows how many people in the U.S. under age 20 were diagnosed between 2017 and 2021 with one of the more common young-people’s cancers. These include blood cancers (such as leukemias and lymphomas) and nervous system cancers (such as brain cancer). Cancers in young people are rare. Even the most common ones only affect between 30 and 50 kids in every 1 million.

Combating cancer

How doctors treat cancer will depend on what type it is, where it is and what stage it is.

Cancers in people under 18 may be treated differently than cancers in adults. The study and treatment of cancer is called oncology.

There are many ways to treat cancer. Often patients will receive more than one type of treatment.

Some early-stage, solid-tumor cancers can be cut out with surgery.

Chemotherapy uses drugs to kill cancer cells or to shrink a tumor. The types of drugs used will depend on the cancer. Other drugs may try to harm cancer cells indirectly. For example, some tumors rely on hormones to grow. For these cancers, doctors might use medicines that stop the body from making or using those hormones.

a young woman with light brown skin is sitting in a hospital bed and smiling for the camera. She's making a V sign with her fingers. She is wearing a pink hospital gown and a white headwrap. She is also covered from the waist down by a blanket. — Doctors might have several different ways to treat a patient’s cancer. But many of those treatments can attack healthy cells, too. So healthy cells that divide a lot — such as hair cells — may also die (allowing a patient’s hair to fall out). Songsak rohprasit/Moment/Getty Images

Some cancers are sensitive to heat or light. Doctors can heat these types of cells with radio waves, lasers or ultrasound. Photodynamic therapy uses a drug to make cancer cells super-sensitive to light. Then doctors can shine a laser or other light source on them to kill those cells.

Radiation, too, can treat cancer. Focused beams of high-energy radiation can kill cancer cells by breaking down their DNA.

Patients with blood cancers may get chemotherapy or radiation to destroy diseased bone marrow. Doctors then infuse marrow from a volunteer so that patients can make healthy new blood cells again. This is called a stem-cell transplant.

Other times, doctors use the patient’s own immune system to fight cancer. Immunotherapy drugs aim to boost the body’s natural immunity. Or these drugs may chemically tag tumors to help the patient’s immune system find and destroy them.

Cancer treatment often has unpleasant side effects. Patients may feel tired or sick to their stomach. That’s because treating a cancer means killing cells — and many of those that get hit may have been healthy ones. That’s especially true for healthy cells that are dividing, such as those lining the stomach, immune cells and hair cells. (That’s why many people can lose their hair during treatment.)

Scientists and doctors are always looking for new ways to treat cancers. Some patients enroll in clinical trials to test new treatments. Although all cancers are different, the goal is the same: Help people whose cells have gotten out of control.

Cancer stages

Doctors often use stages to gauge how advanced a cancer is. Different types of cancer may use different systems. Some are more detailed than others, but many refer to five stages. A higher stage means the cancer is more advanced.

a series of illustrations showing cancer growth from Stage 1 - 4 — Artist’s drawing of a gastrointestinal cancer. In stage I, a tumor develops within the interior layer of the colon’s wall. By stage II, it’s moving toward outer wall. The tumor breaches the organ’s wall by stage III and and its cells are mutating. A stage IV tumor is now seeding tumors at distant sites.Stocktrek Images/Stocktrek Images/Getty Images Plus

Stage 0: Cells are dividing more than they should. Stage 0 is a tumor (for solid-cancer types) but not a true cancer, because the tumor is not ready to spread.

Stage I: The tumor is small and still only in one area. This tumor could spread but has not done so yet.

Stage II: The tumor is larger and bits of it have broken off and spread nearby, perhaps to lymph nodes (part of the immune system). Those small nodes exist all over the body.

Stage III: The original tumor has spread to nearby tissues, as well as to lymph nodes. The spread of cancer cells to distant organs is called metastasis.

Stage IV: The cancer has spread to many parts of the body, far from where it originally formed.

Cancer is always labeled by the stage and tissue where doctors diagnosed it — even if it gets worse afterward.

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What causes cancer?

Types of cancer

Combating cancer

Cancer stages

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