The most common ulcer, a duodenal ulcer, occurs in the first foot (12 inches) of the small intestine after the stomach.  A gastric ulcer is one that occurs in the stomach.  Ulcers are not contagious or cancerous, but ulcers can be a result of cancer in the stomach or pancreas.

Peptic ulcers are caused by three major factors: a bacterium called Helicobacter pylori, which cause almost two-thirds of all ulcers.  Many people have Helicobacter pylori infections, but not everyone who does gets a peptic ulcer.  When people with Helicobacter pylori infections do get ulcers, it is generally because the bacteria weaken the protective coating around the stomach and small intestine.  Then, the acids which the stomach produces to digest food, get to the soft, unprotected tissue lining of the stomach and digestive system. Here the acids and bacteria irritate the tissue until the sores, or ulcers, are formed. 

Other causes are nonsteroidal anti-inflammatory drugs (NSAIDs) such as aspirin (which cause most other ulcers), and other diseases such as acid reflux (which rarely cause ulcers).

Things like stress, spicy foods, alcohol, and smoking cannot cause ulcers but they can aggravate them and make them worse. You’re more likely to get a peptic ulcer if you: have a Helicobacter pylori infection, use NSAIDs often, smoke, drink, are over 50 years old, and have relatives who have peptic ulcers.