Helicobacter pylori is a bacteria without a cell wall that lives in environments with oxygen. First cultured in 1982, it had first been seen in 1875 but it did not grow in blood cultures and scientists were unable to study it effectively.
Prior to the discovery of the bacteria, medicine had assumed gastric ulcers were the result of a combination of stress and a diet of spicy food. However, the bacteria was consistently found in ulcers and Australian experimenters were able to induce gastritis by swallowing samples of the bacteria. Despite initial scepticism, by 1987 the experimenters were able to treat ulcers with a regimen of three different antibiotics for a period of three weeks and by 1994 the NIH was recommending triple antibiotic theory as the preferred treatment.
The discovery of this link won the researchers the 2005 Nobel Prize in medicine & physiology.
Helicobacter is almost unique as being able to thrive in an acidic enviroment that is deadly to most bacteria. Further research has also shown that the bacteria lives in the stomach of most people, but does not cause ulcers.