Animal studies and two small studies in people suggest antibiotics may help prevent a heart attack or complications in people with heart disease. The purpose of this research study is to determine whether an antibiotic, azithromycin, decreases the risk of having a heart attack or other complication related to heart disease in people with heart attack. Azithromycin is an antibiotic that is used to treat infections with a bacteria called Chlamydia pneumoniae. Some studies suggest that Chlamydia pneumoniae infection may be a risk factor for heart attacks and other complications related to heart disease. Chlamydia pneumoniae is spread from person-to-person by sneezing and coughing (unlike a related bacteria, Chlamydia trachomatis, it is not a sexually transmitted disease). Studies show that people who have been infected with Chlamydia pneumoniae in the past have a higher chance of having a heart attack. Other studies show that Chlamydia pneumoniae is present in the plaque, or material blocking the heart vessel, in many people with heart disease. The U.S. Food and Drug Administration (FDA) has approved azithromycin for use in treating many types of infections, including pneumonia and skin infections. The FDA has not approved azithromycin for use in prevention of heart attacks and other complications of heart disease and its use for this purpose is, therefore, considered experimental.