<unk>  colonizes the stomach and induces chronic gastritis, a long-lasting inflammation of the stomach. The bacterium persists in the stomach for decades in most people. Most individuals infected by<unk>  never experience clinical symptoms, despite having chronic gastritis. About 10–20% of those colonized by<unk>  ultimately develop gastric and duodenal ulcers.<unk>  infection is also associated with a 1–2% lifetime risk of stomach cancer and a less than 1% risk of gastric MALT lymphoma. In the absence of treatment,<unk>  infection – once established in its gastric niche – is widely believed to persist for life. In the elderly, however, infection likely can disappear as the stomach's mucosa becomes increasingly atrophic and inhospitable to colonization. The proportion of acute infections that persist is not known, but several studies that followed the natural history in populations have reported apparent spontaneous elimination. It is possible for<unk>  to re-establish in a person after eradication. This recurrence can be caused by the original strain (recrudescence), or be caused by a different strain (reinfection). According to a 2017 meta-analysis by Hu et al., the global per-person annual rates of recurrence, reinfection, and recrudescence is 4.3%, 3.1%, and 2.2% resepctively. It is unclear what the main risk factors are. Mounting evidence suggests<unk>  has an important role in protection from some diseases. The incidence of acid reflux disease, Barrett's esophagus, and esophageal cancer have been rising dramatically at the same time as<unk> s presence decreases. In 1996, Martin J. Blaser advanced the hypothesis that<unk>  has a beneficial effect by regulating the acidity of the stomach contents. The hypothesis is not universally accepted as several randomized controlled trials failed to demonstrate worsening of acid reflux disease symptoms following eradication of<unk> . Nevertheless, Blaser has reasserted his view that<unk>  is a member of the normal flora of the stomach. He postulates that the changes in gastric physiology caused by the loss of<unk>  account for the recent increase in incidence of several diseases, including type 2 diabetes, obesity, and asthma. His group has recently shown that<unk>  colonization is associated with a lower incidence of childhood asthma.
Helicobacter pylori
Giardia duodenalis
Entamoeba histolytica
Neisseria mucosa
Cryptosporidium parvum
Entamoeba gingivalis
Campylobacter upsaliensis
Ancylostoma duodenale