Chlamydia trachomatis (C. trachomatis), better known as chlamydia, is the most common sexually transmitted infection caused by bacteria in the United States. Chlamydia is sometimes called the "hidden infection" because the C. trachomatis bacterium invades a single cell and hides there, gaining energy from the cell. This STI is easy to treat with antibiotics, but many of the people who have it show no symptoms at all, which is one reason it is so common. Left untreated, chlamydia can lead to complications. Readers will find a wealth of information about the infection, including the science behind the illness, how to treat it, and of course, how to avoid it. Sidebars and detailed diagrams help readers make the most of this essential guide to sexual health.