In the United States, one out of 10 people develops stomach ulcers at some point in life.
Stomach acid breaks down food, but it can also digest the stomach wall itself.
Ulcers result when there's a breakdown in the stomach's protective lining of mucous. It can cause pain and, apparently in Boris Yeltsin's case, internal bleeding. Hemorrhaging is a common, but serious ulcer complication that can lead to life-threatening blood loss. Signs of bleeding may be present on either end of the digestive tract. Doctors often look for the source of bleeding using an endoscope, a long lighted tube that is passed through the throat into the stomach.