Basic Facts

What is Cancer?

When cells grow normally in your body, they divide into exact copies of themselves.  When there are enough cells needed for a particular part of the body, the cells stop multiplying. 

Sometimes cells grow abnormally. Often, this isn’t dangerous, and the cells grow into a clump of cells known as a benign tumor (similar to a wart). These benign tumors grow slowly. Often they can be easily removed surgically, or left alone.

But at other times, cells start to grow in an “out-of-control” way, invading other parts of the body. This type of abnormal cell growth is called cancer. Many cancer cells together form a clump or mass called a malignant tumor. Other cancers do not form tumors. (For example, leukemia is a cancer of the bone marrow and blood.) Cancer can also spread from its original location to another part of the body. This is called metastatic cancer.

Although our bodies have ways of detecting and repairing abnormal cells, these processes often don’t work as well when we get older. Our immune systems normally destroy old, damaged, or abnormal cells. However, our immune systems are also less effective as we age. For these reasons, cancer is primarily a disease of older adults. In the US, half of all cancers occur in people over the age of 65.

The medical study of cancer and its treatment is called oncology. Physicians who specialize in cancer are known as oncologists.

How Common is Cancer?

Half of all men and one-third of all women in the US will be diagnosed with cancer sometime during their lifetimes.  After heart disease, it is the leading cause of death in the US. The most common cancers are lung cancer, colon cancer, breast cancer (for women), and prostate cancer (for men). Some cancers – most notably, breast cancer -- are less aggressive when they occur in older adults.

In the last few years, there has been a decline in cancer deaths for both men and women.  The overall rate of new cancer diagnoses annually has also decreased. This promising sign is due to improvements in cancer prevention, detection, and treatment. 


Last Updated June 2020