When you're told that you have stomach cancer, it's natural to wonder what may have caused the disease. But no one knows the exact causes of stomach cancer. Doctors seldom know why one person develops stomach cancer and another doesn't.

Doctors do know that people with certain risk factors are more likely than others to develop stomach cancer. A risk factor is something that may increase the chance of getting a disease.

Studies have found the following risk factors for stomach cancer:

• Helicobacter pylori infection: H. pylori is a bacterium that commonly infects the inner lining (the mucosa) of the stomach. Infection with H. pylori can cause stomach inflammation and peptic ulcers. It also increases the risk of stomach cancer, but only a small number of infected people develop stomach cancer. You may want to read the NCI fact sheet H. pylori and Cancer.
• Long-term inflammation of the stomach: People who have conditions associated with long-term stomach inflammation (such as the blood disease pernicious anemia) are at increased risk of stomach cancer. Also, people who have had part of their stomach removed may have long-term stomach inflammation and increased risk of stomach cancer many years after their surgery.
• Smoking: Smokers are more likely than nonsmokers to develop stomach cancer. Heavy smokers are most at risk.
• Family history: Close relatives (parents, brothers, sisters, or children) of a person with a history of stomach cancer are somewhat more likely to develop the disease themselves. If many close relatives have a history of stomach cancer, the risk is even greater.
• Poor diet, lack of physical activity, or obesity:
  • Studies suggest that people who eat a diet high in foods that are smoked, salted, or pickled have an increased risk for stomach cancer. On the other hand, people who eat a diet high in fresh fruits and vegetables may have a lower risk of this disease.
  • A lack of physical activity may increase the risk of stomach cancer.
  • Also, people who are obese may have an increased risk of cancer developing in the upper part of the stomach.

Most people who have known risk factors do not develop stomach cancer. For example, many people have an H. pylori infection but never develop cancer.

On the other hand, people who do develop the disease sometimes have no known risk factors.

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