Cancer is not really one disease, but rather a collection of 200 related diseases that occur throughout the body.  While each form of cancer is unique, all cancers involve the abnormal and uncontrolled growth of cells.  A normal cell is genetically programmed to divide a certain number of times and then die. This ensures the preservation of the cell's genetic material.  Cells also stop growing when signals from neighboring cells tell them to stop dividing because they have formed a monolayer, or single sheet, across a certain surface.   However, when a cell becomes mutated, it does not sense signals from its neighbors and continues to divide, despite creating clumps of cells in certain places.  This is the beginning of what is commonly called a tumor.  Although not all tumors are bad, every tumor has the potential to be cancerous.  (See the page on how cancer spreads for more information.)  Cancerous cells also lack the ability to degenerate and die after a certain number of divisions.  This also causes a problem because in addition to having too many cells in one place, the cells do not die as normal cells do. 

Cancer can be caused by many factors, including: genetic flaw, exposure to cancer-causing agents (called carcinogens), cigarette smoke, poor diet and inactivity, radiation, viruses, sunlight, and other unknown causes.  Because every part of our body is comprised of cells, cancer can occur in virtually any part of the body.  Some of the major forms of cancer include: leukemia, melanoma, breast cancer, pancreatic cancer, lung cancer, and brain cancer. 

Leukemia is a form of cancer that manifests itself in the white blood cells.  Melanoma is the most severe form of skin cancer, which is mainly caused by excessive exposure to the sun.  Breast cancer begins as a lump or thickening in the breast tissue and can spread quickly to the lymph system.  Pancreatic cancer interferes with the normal function of the pancreas, which makes insulin to break down sugars in the body.  This form of cancer is nearly always terminal.  Lung cancer is mainly caused by cigarette smoke and inhalation of other carcinogens, usually causing many tumors to form in one or both lungs.  Brain cancer can be highly detrimental to an individual, causing loss of motor function and poor blood flow to other parts of the brain because as the tumor grows, it damages normal cells and starves them of nutrients.