What is cancer?

Cancer is not just one disease – there are many types of cancer. It can start in different places in the body. Cancer can start in the lungs, the breast, the colon, or even in the blood. Cancers are alike in some ways, but they can be different in the ways they grow and spread.

How are cancers alike?

The cells in our bodies all have their own jobs to do. Normal cells divide in a certain way. When they’re worn out or damaged, they’re replaced with new cells. Cancer is a disease in which cells aren’t normal. Cancer cells keep on growing out of control, making new cells that don’t die. These cells crowd out normal cells. This causes problems in the part of the body where the cancer started.

Cancer cells can also spread to other parts of the body and cause problems in those places. For instance, cancer cells in the lung can travel to the bones and grow there. When cancer cells spread, it’s called metastasis (meh-TAS-tuh-sis). But when lung cancer cells start to grow in the bones, it’s still called lung cancer. Cancers are named for where they start, not where they end up.

How are cancers different?

Some cancers tend to grow and spread very quickly. Others grow more slowly. They also respond to treatment in different ways. Some types of cancer are best treated with surgery. Others do better with drugs or radiation treatments. Often 2 or more treatments are used to get the best results. (We talk about this in the section called “ Common types of cancer treatment.”)

When someone has cancer, the doctor will want to find out what kind of cancer it is. People with cancer need treatment that’s aimed at their type of cancer.

The American Cancer Society medical and editorial content team
Our team is made up of doctors and master’s-prepared nurses with deep knowledge of cancer care as well as journalists, editors, and translators with extensive experience in medical writing.

Last Medical Review: February 20, 2015 Last Revised: February 20, 2015

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