Helicobacter pylori is a Gram-negative, spiral-shaped, flagellated pathogenic bacterium that infects the protective mucus that lines the stomach. It is very possibly the most common chronic infection in humans worldwide. Though some people suffer no ill effects from it, initial acute infections typically cause inflammation of the stomach (gastritis) which results in abdominal pain, loss of appetite, nausea, and vomiting. Chronic infections can lead to peptic ulcers or possibly stomach cancer (gastric adenocarcinoma).
Many scientists believe that H. pylori is responsible for all non-chemical-related cases of stomach ulceration, and the World Health Organization (WHO) has declared the microbe to be a Class 1 carcinogen. The infection can be cured and pre-cancerous tumors reversed with antibiotic treatment.
From the science dictionary at http://biotech.icmb.utexas.edu/.