Helicobacter pylori is a spiral Gram-negative bacterium (2-6.5 µm in size, flagellated) that colonizes the human gastric mucosa. The organism is found in the mucus layer and adheres to the surface mucus epithelium of the stomach but generally does not penetrate the gastric mucosa directly.

However, there is a secondary inflammatory response in the mucosa leading to chronic active gastritis. Helicobacter pylori are the primary causative agent in most cases of peptic ulcer disease. In 1994 the WHO classified Helicobacter pylori as a category 1 carcinoma. The infection rate in Europe is about 30%-40%, and worldwide about 50%. There is an inverse relationship between the presence of Helicobacter pylori infection and socioeconomic status.

In developing countries, people acquire the infection at an early age such that by young adulthood as many as 90% of the population might have Helicobacter pylori gastritis. In developed western countries the prevalence of Helicobacter pylori gastritis is much lower. Under these conditions, the rate of acquisition is much slower (roughly 1% per annum) and the older one is, the more likely one is to be infected with the organism