Cancer is a large group of over one hundred diseases characterized by the uncontrolled growth and spread of abnormal cells, rather than following normal cell division and death cycles. These faulty cells can form tumors, invade nearby tissues, and spread to other parts of the  body via the blood stream or lymphatic system causing severe illness.
Cancer is always a genetic disease because it is caused by mutations to genes that control cell growth, but only about five to ten percent of cancers are hereditary. Most cancers are caused by somatic mutations stemming from aging, lifestyle factors, or environmental exposures rather than inherited genes.

Cancer is the leading cause of death worldwide but it is not inherently fatal. While it is the second most common cause of death in the U.S. many cancers are curable if detected early. Survival rates have doubled in the last forty years with roughly fifty percent of people surviving for ten years or more. 

The most common cancers worldwide and in the U.S. are female breast, prostate, lung, and colorectal cancers, with breast cancer often being the  most common by incidence overall. Lung cancer is the leading cause of cancer death while non melanoma skin cancer is highly prevalent but often tracked separately.