Cancer is when cells in your body grow out of control

Cancer happens when cells in your body start to divide and multiply in a way they should not. Normally, your cells grow, divide to replace old or damaged cells, and then die in an orderly way. With cancer, this process breaks down. Cells keep dividing even when your body does not need new ones, and they do not die when they should. Over time, these extra cells pile up and form a lump or tumor, or they spread through your blood and lymph system to other parts of your body.

Cancer is not one disease — it is many diseases. A cancer is named for the type of cell it starts in and the part of the body where it begins. Lung cancer starts in the lungs. Breast cancer starts in breast tissue. Colon cancer starts in the colon. The same type of cancer can behave differently in different people, and the same person can develop more than one type of cancer.

Key Takeaways

  • Cancer occurs when cells divide uncontrollably instead of following the body's normal growth and death cycle.
  • A tumor is a mass of cancer cells, but not all tumors are cancer — some are benign (noncancerous) and do not spread.
  • Cancer is named for the type of cell it starts in and where in the body it begins, such as lung cancer or breast cancer.
  • Cancer cells can spread to other parts of the body through the bloodstream and lymph system in a process called metastasis.
  • Risk factors like age, family history, smoking, and sun exposure can increase the chance of developing cancer, but having a risk factor does not mean you will get cancer.

How cells normally work versus how cancer cells work

Every cell in your body has a set of instructions written in its DNA. These instructions tell the cell when to divide, when to stop dividing, and when to die. Your body uses this system to keep the right number of cells in each tissue and organ. When you cut your skin, cells divide to heal the wound. When cells get old or damaged, they receive a signal to die so new cells can take their place.

Cancer cells ignore these instructions. A cancer cell may have damage to its DNA that tells it to keep dividing no matter what. Or it may have lost the ability to hear the signal that says "stop dividing" or "die now." Some cancer cells also learn to hide from your immune system, which normally patrols your body looking for cells that do not belong. Over time, cancer cells can become better at surviving and spreading, which is why cancer often gets harder to treat the longer it goes untreated.

The difference between tumors and cancer

A tumor is simply a lump or mass of cells. Not all tumors are cancer. A benign tumor is made of cells that do not spread to other parts of the body and usually do not threaten your life. Benign tumors can still cause problems — they might press on nerves or blood vessels, or they might produce hormones that throw your body out of balance — but they stay in one place.

A malignant tumor is cancer. Malignant means the cells can spread to other parts of your body. When cancer cells break away from the original tumor and travel through your blood or lymph system to start new tumors elsewhere, doctors call this metastasis. A cancer that has spread is called metastatic cancer. The original tumor is called the primary cancer, and new tumors that form from cells that spread are called secondary cancers.

What causes cells to become cancerous

Cancer develops when cells accumulate damage to their DNA over time. This damage can come from many sources. Some people are born with a genetic change that makes cancer more likely — these are called hereditary cancers. Others develop cancer because of things that happen in their lifetime: smoking, heavy sun exposure, certain infections, obesity, or heavy alcohol use. Many cancers have no single clear cause; instead, they result from a combination of age, genetics, and environmental factors.

It usually takes years or decades for enough damage to build up in a cell for it to become cancerous. This is why cancer is more common as people get older. It is also why reducing your exposure to known risk factors — not smoking, protecting your skin from the sun, limiting alcohol, maintaining a healthy weight — can lower your risk. Having a risk factor does not mean you will get cancer, and not having any known risk factors does not mean you will not.

How doctors find cancer

Doctors find cancer in several ways. Sometimes a person notices a symptom — a lump, bleeding, persistent cough, or pain — and goes to the doctor, who then runs tests. Other times, cancer is found during a screening test done on people without symptoms, such as a mammogram for breast cancer or a colonoscopy for colon cancer. Screening tests look for early signs of cancer before a person feels sick.

Once a doctor suspects cancer, they usually order imaging tests like X-rays, ultrasound, CT scans, or MRI scans to see inside the body. To confirm cancer, doctors almost always need a biopsy — a small sample of tissue taken from the suspicious area and looked at under a microscope. The biopsy shows whether the cells are cancer, what type of cancer they are, and sometimes how fast they are likely to grow.

Stages of cancer and what they mean

After cancer is found, doctors assign it a stage. The stage describes how much cancer is in the body and where it is. Staging helps doctors decide on treatment and gives an idea of what to expect. Most cancers are staged from 1 to 4, though the exact system varies by cancer type.

Stage 1 usually means the cancer is small and has not spread. Stage 2 often means the cancer is larger or has spread to nearby lymph nodes. Stage 3 typically means the cancer has spread further into nearby tissues or lymph nodes. Stage 4 means the cancer has spread to distant parts of the body. A lower stage generally means the cancer may be easier to treat, but treatment depends on many factors beyond stage alone, including the person's age and overall health.

Types of cancer and where they start

Cancers are grouped by the type of cell they start in. Carcinomas start in cells that cover the inside or outside of organs — these are the most common type and include breast, lung, colon, and skin cancers. Sarcomas start in bone, muscle, fat, or other connective tissue. Leukemias start in blood-forming cells in the bone marrow and do not form solid tumors. Lymphomas start in immune system cells called lymphocytes and often begin in the lymph nodes.

Within each broad type, there are many subtypes. For example, lung cancer can be non-small cell lung cancer or small cell lung cancer, and each type behaves differently and responds to different treatments. Knowing the exact type of cancer is important because it guides which treatments are most likely to work.

Frequently Asked Questions

Is cancer contagious?

No, cancer is not contagious. You cannot catch cancer from another person through touch, breathing the same air, sharing food, or any other contact. Cancer starts from a person's own cells and cannot spread from one person to another. Some infections that increase cancer risk — like human papillomavirus (HPV) or hepatitis B — are contagious, but the cancer itself is not.

Can you have cancer and not know it?

Yes. Many cancers do not cause symptoms in their early stages. Some people have cancer for months or years before they notice anything wrong. This is why screening tests for common cancers — like mammograms, colonoscopies, and Pap smears — are recommended even when you feel fine. Screening can find cancer before symptoms appear, when treatment is often more effective.

Does a family history of cancer mean I will get cancer?

No. Having a family member with cancer increases your risk, but it does not mean you will develop cancer. Risk depends on which family member had cancer, what type it was, how old they were when diagnosed, and whether a genetic mutation runs in your family. A doctor or genetic counselor can help you understand your personal risk based on your family history.

What is the difference between cancer and a cyst?

A cyst is a fluid-filled sac that forms in the body. Most cysts are benign and do not need treatment. Cancer is a disease of cells that divide uncontrollably. A cyst is not cancer, though imaging tests are sometimes needed to confirm that a lump is a cyst and not something else. Your doctor can explain what imaging shows about any lump you have.

Can cancer go away on its own?

Very rarely, a person's immune system may fight off early cancer cells before they form a tumor. However, once cancer has formed and been diagnosed, it almost never goes away without treatment. This is why early detection and prompt treatment are important. If you have been diagnosed with cancer, your doctor will discuss treatment options with you.