Once started, cancer can grow around and through neighboring organs and tissues, blocking blood vessels and crowding other organs out of position. It can spread in another way, too, when cancerous cells break away from the original growth. These cells can travel through the bloodstream or the lymphatic system to other areas in the body. Once they find a place where they can attach, they can start to divide again, and a new cancerous growth is formed. If it is not stopped by treatment, cancer can kill.
Cancer is actually a group of many related diseases.
It can occur at any age, although it is more common after age 40. The exact causes of cancer have yet to be proven. But researchers do know that cancer is linked to personal habits, such as smoking, drinking alcohol and poor diet. It also is linked to environmental factors, including ultraviolet sunlight, chemicals in air pollution and elsewhere, and occupational hazards such as overexposure to x-rays and industrial chemicals like asbestos. Some types of cancer seem to track in families.