Cancer- An overview

Doctors often cannot explain why one person develops cancer and another does not. But research shows that certain risk factors increase the chance that a person will develop cancer. These are the most common risk factors for cancer:
Growing older
Tobacco
Sunlight
Ionizing radiation
Certain chemicals and other substances
Some viruses and bacteria
Certain hormones
Family history of cancer
Alcohol
Poor diet, lack of physical activity, or being overweight
Many of these risk factors can be avoided. Others, such as family history, cannot be avoided. People can help protect themselves by staying away from known risk factors whenever possible.
If you think you may be at risk for cancer, you should discuss this concern with your doctor. You may want to ask about reducing your risk and about a schedule for checkups.
Over time, several factors may act together to cause normal cells to become cancerous. When thinking about your risk of getting cancer, these are some things to keep in mind:
Not everything causes cancer.
Cancer is not caused by an injury, such as a bump or bruise.
Cancer is not contagious. Although being infected with certain viruses or bacteria may increase the risk of some types of cancer, no one can "catch" cancer from another person.
Having one or more risk factors does not mean that you will get cancer. Most people who have risk factors never develop cancer.
Some people are more sensitive than others to the known risk factors.
The sections below have more detailed information about the most common risk factors for cancer. You also may want to read the NCI booklet Cancer and the Environment.
Growing OlderThe most important risk factor for cancer is growing older. Most cancers occur in people over the age of 65. But people of all ages, including children, can get cancer, too.
Tobacco
Tobacco use is the most preventable cause of death. Each year, more than 180,000 Americans die from cancer that is related to tobacco use.
Avoid tobacco.
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Using tobacco products or regularly being around tobacco smoke (environmental or secondhand smoke) increases the risk of cancer.
Smokers are more likely than nonsmokers to develop cancer of the lung, larynx (voice box), mouth, esophagus, bladder, kidney, throat, stomach, pancreas, or cervix. They also are more likely to develop acute myeloid leukemia (cancer that starts in blood cells).
People who use smokeless tobacco (snuff or chewing tobacco) are at increased risk of cancer of the mouth.

Source: http://www.cancer.gov/cancertopics/wyntk/overview/page4