Although scientists still do not fully understand why some people get gastric cancer and others do not, Helicobacter pylori, the bacterium linked to ulcers, has been implicated in stomach cancers.

H. pylori may also be a cause or co-factor for gastric cancer, as its presence increases the risk of developing stomach tumours.

H. pylori infection is recognized as a causal factor in the pathogenesis of chronic gastritis, peptic ulcer, gastric cancer, and gastric MALT lymphoma.