The bacterium Helicobacter pylori, a known cause of stomach ulcers, has also been linked to the occurrence of rare stomach cancers. Strains of H. pylori with the cagA gene make breakdown of the stomach lining and precancerous changes to the gastrointestinal system more likely. About 10% of patients with breakdown of the stomach lining develop cancer within 10 years. H. pylori is thought spread via human contact. It is present in the stomach for life, and is present in about 10% of healthy people under 30 years old. Other factors, such as diet and smoking behaviour, also contribute to cancer.