Over the past year, evidence has been accumulating that molecular mimicry, in which immune responses triggered by an invading pathogen mistakenly attack the body9s own tissues, cause diseases such as an eye inflammation and Lyme arthritis. Results reported on page 1335 now suggest that one of the most common serious illnesses--heart disease--may be added to the list. Working in mice, the researchers have shown that Chlamydia bacteria, which cause infections of the genital and respiratory tracts, carry a peptide that resembles one in the heart protein myosin and triggers a strong inflammatory attack on the heart. If something similar occurs in human beings and the inflammation also plays a role in the formation of the artery-clogging plaques of atherosclerosis--two big ifs--the work may provide a molecular explanation for a long-suspected link between infections and heart disease.