Cancer is a broad class of diseases involving out-of-control cell growth. Cancerous cells rapidly divide and grow, producing masses of tissue called tumors. While benign tumors cannot metastasize, or invade neighboring tissue, and so are relatively harmless, malignant tumors can spread throughout the body, destroying healthy tissue and interfering with the regular function of bodily systems. Historically, people have often referred to cancer as though it was a single disease. In fact, there are over 200 different cancers, all originating within different cells in different organs throughout the body. This extraordinary diversity is one of the reasons that it remains so difficult to understand, prevent, diagnose, and treat cancers.
The causes of cancer are complex and not fully understood. Some cancers can be traced directly to inherited genetic traits, but many more cannot. A wide range of environmental, dietary, and behavioral factors, as well as some infections, are known to increase risk of cancerous cell mutations. The most significant risk factor for cancer is old age.
Cancer is a relatively common type of disease--nearly 14 million people living in the United States have been diagnosed with a cancer at some point in their lives. It has been estimated that over 1.6 million Americans were diagnosed with cancer in 2014 alone. While incidence rates have increased over the last few decades, largely due to an aging population, advances in treatment have significantly improved survival rates.