Infection of the stomach with H. pylori is not the cause of illness itself; over half of the global population is infected but most are asymptomatic.[7][8] Persistent colonization with more virulent strains can induce a number of gastric and extragastric disorders.[9] Gastric disorders due to infection begin with gastritis, inflammation of the stomach lining.[10] When infection is persistent the prolonged inflammation will become chronic gastritis. Initially this will be non-atrophic gastritis, but damage caused to the stomach lining can bring about the change to atrophic gastritis, and the development of ulcers both within the stomach itself or in the duodenum, the nearest part of the intestine.[10] At this stage the risk of developing gastric cancer is high.[11] However, the development of a duodenal ulcer has a lower risk of cancer.[12]Helicobacter pylori is a class 1 carcinogen, and potential cancers include gastric mucosa-associated lymphoid tissue (MALT) lymphomas and gastric cancer.[10][11] Infection with H. pylori is responsible for around 89 per cent of all gastric cancers, and is linked to the development of 5.5 per cent of all cases of cancer worldwide.[13][14]H. pylori is the only bacterium known to cause cancer.[15]
Extragastric complications that have been linked to H. pylori include anemia due either to iron-deficiency or vitamin B12 deficiency, diabetes mellitus, cardiovascular, and certain neurological disorders.[16] An inverse link has also been claimed with H. pylori having a positive protective effect on many disorders including asthma, esophageal cancer, IBD (including GERD and Crohn's disease) and other disorders.[16]
Some studies suggest that H. pylori plays an important role in the natural stomach ecology by influencing the type of bacteria that colonize the gastrointestinal tract.[17][18] Other studies suggest that non-pathogenic strains of H. pylori may beneficially normalize stomach acid secretion, and regulate appetite.[19]