Cancer , any of more than 100 diseases characterized by excessive, uncontrolled growth of abnormal cells, which invade and destroy other tissues. Cancer develops in almost any organ or tissue of the body, but certain types of cancer are more life-threatening than others. In the United States and Canada cancer ranks as the second leading cause of death, exceeded only by heart disease.
For reasons not well understood, cancer rates vary by gender, race, and geographic region. For instance, more men than women develop cancer, and African Americans are more likely to develop cancer than people of any other racial group in North America. The frequency of certain cancers also varies globally. For example, breast cancer is more common in wealthy countries, and cervical cancer is more common in poor countries.
Although people of all ages develop cancer, most types of cancer are more common in people over the age of 50.
Cancer usually develops gradually over many years, the result of a complex mix of environmental, nutritional, behavioral, and hereditary factors. Scientists do not completely understand the causes of cancer, but they know that certain lifestyle choices can reduce the risk of developing many types of cancer. Not smoking, eating a healthy diet, and exercising moderately for at least 30 minutes each day can lower the likelihood of developing cancer.
Just 60 years ago a cancer diagnosis carried little hope for survival because doctors understood little about the disease and how to control it. Today about two-thirds of all Americans diagnosed with cancer live longer than five years. While it is difficult to claim that a cancer patient is disease free, long-term survival significantly improves if the patient has had no recurrence of the cancer for five years after the initial diagnosis. For years, death rates from cancer were rising in developing countries. In 2006 the American Cancer Society reported that the number of cancer deaths in the United States dropped for two years in a row. The decrease was attributed to a decline in smoking, earlier detection, and improved treatment.