Seen here via immunohistochemical staining of a gastric biopsy is the Heliobacter Pylori bacteria, or H. Pylori if you're in a hurry.
Able to survive the intensely acidic environment that is the human stomach, H. Pylori actually gets downright comfortable there. The bacterium has flagella and moves through the stomach lumen, ultimately drilling into the stomach's mucoid lining. Once safely ensconsed, H. Pylori quickly colonizes.
Colonization of the stomach by H. pylori results in chronic gastritis, an inflammation of the stomach lining. The severity of the inflammation is likely to underlie H. pylori-related disease.
At least half the world's population are infected by the bacterium, making it the most widespread infection in the world.