In past centuries people feared epidemic diseases with their sudden onset, ghastly symptoms, agonizing death for many, and sometimes disfigurement or physical impairment for survivors. Today, especially in the developed world (with a few notable exceptions), the dread of epidemic contagion seems almost as anachronistic as the burning of witches. It has been replaced by the dread of cancer. As with the epidemics of yesterday, the basic causes of cancer remain shrouded in mystery, while its effects in terms of human suffering are all too well known.

Cancer is a process whereby a loss of control of normal cell division and multiplication produces a tumor that can invade adjacent tissues and metastasize, that is, implant cancerous cells at a site that is noncontiguous to their origin, where abnormal multiplication continues. When cancer originates in connective tissues (mainly bone or muscle), it is called sarcoma; when it originates in epithelial tissues (lining tissues and organs such as the breast, lungs, or stomach), it is called carcinoma. The latter is by far more common. Invasive tumors occur in all complex species and probably antedate the advent of vertebrates. The oldest paleopathological evidence is limited to lesions that affected bones, such as those found in dinosaurs. Tumors have been found in Egyptian mummies dating from 2000 to 3000 b.c., and physicians of that ancient land knew of and treated patients for cancers of several sites.