Cancer 101 | National Geographic

• [Narrator] Today cancer causes one in every seven deaths worldwide. But how does cancer start, and what is being done to combat it? Our bodies contain trillions of highly specialized cells, and each carries genes responsible for regulating cell growth and division. But when a genetic change disrupts this process, cells begin to grow and divide uncontrollably, thereby becoming cancer.

The genetic changes that cause cancer usually happen in three types of genes: proto-oncogenes, which signal a cell to grow and divide; tumor suppressor genes, which signal a cell to stop dividing; and DNA repair genes, which preserve and maintain genetic codes. Changes in these genes lead to abnormal cell growth, resulting in masses of tissues called tumors. Tumors may be benign, meaning that they remain in one area, or malignant, which means that they are capable of spreading.

Through a process called metastasis, a malignant tumor's cells will eventually break off, travel throughout the body, and begin forming tumors in other regions. When this happens, the cancer has become metastatic and is most dangerous. The cause of cancer is still a mystery. Inherited traits may be behind some of the genetic changes that lead to the formation of cancerous cells, and so might environmental exposures, such as excessive radiation from the sun or chemicals in cigarette smoke.

In the United States today, roughly one-third of all people will eventually develop cancer. Of the over 100 different types of cancer, breast cancer is currently the most prevalent type, and lung cancer causes the most deaths. But our ability to fight cancer is improving. Since just the 1990s, cancer mortality rates have dropped more than 26% in the U.S., and more than two million lives have been saved.

As treatments and methods of early detection improve, and doctors, scientists, and the public better understand cancer, hope remains in the fight against the disease.