Ask the average American about chlamydia, and you will probably evoke an uneasy cringe. Most people think immediately of one of the world's most common sexually transmitted diseases (STDs). But the term actually refers to an entire genus of tiny bacteria that can ignite a variety of serious illnesses. Ask a poor mother in Africa about chlamydia, and she may tell you that flies transmitting this infection gave her two young children the painful eye condition known as conjunctivitis. This illness--caused by a strain of Chlamydia trachomatis (the species that also causes STDs)--can lead to trachoma, a potentially blinding disease. In industrial countries, an airborne species, C. pneumoniae, causes colds, bronchitis and about 10 percent of pneumonias acquired outside of hospitals. Researchers have even drawn tentative links between C. pneumoniae and atherosclerosis, the artery-narrowing condition that leads to heart attacks and strokes.

Because chlamydiae are bacteria, antibiotics can thwart the infections they produce. Unfortunately, the illnesses often go undetected and untreated, for various reasons. The genital infections rarely produce symptoms early on. And in developing countries where trachoma is a concern, people often lack access to adequate treatment and hygiene. As a result, many of the estimated 600 million people infected with one or more Chlamydia strains will go without medical care until the consequences have become irreversible. It is unrealistic to expect that doctors will ever identify all individuals who have the STD or that improved hygiene will soon wipe out the trachoma-causing bacteria in developing countries. For these reasons, the best hope for curtailing the spread of these ailments is to develop an effective vaccine or other preventive treatments. To discover agents able to block infections before they start, scientists need to know more about how chlamydiae replicate, incite disease and function at a molecular level. But that information has been hard to come by. These bugs are wily. Not only do they have varied strategies for evading the body's immune system, they also are notoriously difficult to study in the laboratory. In the past five years, however, new research--including the complete sequencing of the genomes of several Chlamydia strains--has helped scientists begin to address these obstacles. The resulting discoveries are renewing hope for developing new prevention strategies.