H. pylori infection is closely associated with four diseases of the upper gastrointestinal tract: (i) chronic gastritis; (ii) peptic ulcer; (iii) MALT lymphoma; and (iv) gastric cancer. Hp infection may play a role in the development of coronary heart disease, hypertension, cerebrovascular disease, immune disorders, nutritional and metabolic diseases, and skin diseases. Hp is found in the gastric mucosa and breaks down urea to produce ammonia. Animal experiments have found that live Hp bacteria in the stomach of cirrhotic rats can affect blood ammonia concentrations, with increased blood ammonia levels and reduced survival in Hp-infected cirrhotic rats compared to controls. However, in studies on humans, Hp infection or not appeared to have no effect on blood ammonia and the development of hepatic encephalopathy. It is not clear whether Hp infection is associated with hepatocarcinogenesis. Never before has a causative agent been found to be associated with so many diseases as Hp, and the effectiveness of eradication therapy is even more encouraging. Further research will certainly advance the understanding of the pathogenesis and treatment modalities of Hp and extra-gastrointestinal diseases.