Cancer is a disease that starts when cells in the body grow and divide without control or order. This happens when the cell’s DNA changes in a way that prevents it from making normal copies of itself. Then the abnormal cells start to grow out of control and can form a mass, or tumor. The cancer cells can also spread to other parts of the body through the blood or lymph systems. They can also continue growing and spreading until it forms a new cancer elsewhere in the body, which is called metastasis. There are more than 100 types of cancer that affect different cells, organs or tissues in the body. Some cancers are named for the part of the body they begin in, such as breast, lung and brain cancers. Others are named for the kind of cell they start in, such as sarcomas, lymphomas and melanoma. Cancer treatments can help stop cancer cells from growing or spreading by killing them, removing them or slowing their growth. Treatment can also help control symptoms caused by the cancer, such as pain and bleeding. Some kinds of cancer can come back after treatment and may need to be treated again.

Cancers usually start in cells that make up organs and other tissues, such as skin, bone, muscles and blood. Normally, cells grow and divide to replace old or damaged cells that have died. Cells also die by design in a process called apoptosis. Sometimes these natural processes go wrong, and cancer cells start to grow out of control and form a tumor or malignant mass.

In some cases, the cancer cells don’t die when they should and keep growing to crowd out normal cells in the area. As the tumor grows, it can push on nearby structures such as nerves, blood vessels and other organs. Some tumors are benign, which means they are not cancer and don’t spread. Others are cancer, which is why doctors can tell the difference by taking a sample of the tissue and testing it for cancer cells.

There are many things that can cause cancer, including some habits and lifestyle choices, such as smoking. Other causes of cancer include inherited genes that increase the risk for certain cancers and substances in the environment that can cause cancer by damaging cells. These substances are called carcinogens.

The information in this PDQ is based on research from medical journals. It is written for patients, their families and caregivers. This PDQ was last reviewed by the National Cancer Institute (NCI). This information does not replace the advice of your doctor or other health care provider. We encourage you to talk about any questions or concerns you have with your provider.