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. Certain types of H. pylori have greater risks than others. Other common causes include eating pickled vegetables and smoking. Smoking increases the risk of developing gastric cancer significantly, from 40% increased risk for current smokers to 82% increase for heavy smokers. Gastric cancers due to smoking mostly occur in the upper part of the stomach near the esophagus.