Helicobacter pylori is a type of bacteria that colonizes the stomach of humans and other animals. It is a Gram-negative, spiral-shaped organism that is the leading cause of chronic gastritis, peptic ulcer disease, and gastric cancer. The bacteria can also cause a wide range of other gastrointestinal diseases, including duodenal ulcers, gastric ulcers, and gastric cancer. H. pylori is most commonly transmitted through contaminated food and water. It can also be transmitted from person to person through saliva or vomit, but this is less common. The bacteria are able to survive in the acidic environment of the stomach, where they attach to the gastric mucosal cells and secrete toxins that damage the epithelial cells. Research has shown that H. pylori is a major risk factor for the development of gastric cancer, and that it is responsible for approximately 20-50% of gastric cancer cases worldwide. H. pylori infection is also associated with an increased risk for other gastric diseases, such as gastritis and peptic ulcer disease.