Heart attacks result when blockages form in the arteries that supply blood to the heart (the coronary arteries). Chronic infection with certain bacteria may be among the many factors that contribute to these blockages, possibly by causing inflammation of the blood vessel walls, which damages the walls and allows blockages to form. Chronic infections like this do not cause people to feel sick but can be detected using blood tests that tell whether the person has been exposed to the infection in the past. Helicobacter pylori (H. pylori) is a type of bacteria that appeared to be associated with coronary artery disease in some (but not all) previous studies; however, those studies could not prove whether the H. pylori infection came before or after the heart disease. Moreover, although H. pylori infection is very common in general, it is most common in people who are poor. In past studies, it was unclear whether heart disease was associated with H. pylori infection itself or other risk factors for heart disease that are more common among people who are poor.