H. pylori is a spiral-shaped pathogenic bacterium that infects the protective mucus that lines the stomach. Though some people suffer no ill effects, initial infections typically cause chronic gastritis and can lead to peptic ulcers or possibly stomach cancer (gastric adenocarcinoma). Many scientists believe that H. pylori is responsible for all non-medication-related cases of stomach ulceration, and the World Health Organization (WHO) has declared the microbe a Class 1 carcinogen. The infection can be cured and pre-cancerous tumors reversed with antibiotic treatment.