Cancer is a disease in which cells begin to divide when they shouldn’t, producing masses of cells called tumors. Cancerous tumors are called “malignant” ones because of their ability to grow aggressively, invade surrounding healthy tissue, and “metastasize”, or spread to different locations in the body as tumor cells break off, travel through the blood or lymph, and establish new tumors.
Cancer is generally caused by environmental agents known as “carcinogens”, which include chemicals (such as those found in cigarette smoke) and radiation (like the UV rays of sunlight) that mutate cells, causing them to become cancerous.
Tumors that originate from epithelial cells are generally known as carcinomas, and it is estimated that about 90% of all human cancers are carcinomas. Tumors that originate from muscle tissue are called “sarcomas”, and tumors can also originate from nervous and connective tissues. Epithelial tissue exists in various locations in the body, including the skin, the mucous membranes lining the lungs and gastrointestinal tract, and the linings surrounding major organs.
The observation that 90% of human cancers are carcinomas implies that: