Stomach cancer, also called gastric cancer, is a cancer that starts in the stomach. To understand stomach cancer, it helps to know about the normal structure and function of the stomach. The most common cause is infection by the bacterium Helicobacter pylori, which accounts for more than 60% of cases. Most cases of stomach cancers are gastric carcinomas. Lymphomas and mesenchymal tumors may also develop within the stomach. Most of the time, stomach cancerdevelops through a number of stages over a number of years.