The bacterium Chlamydia trachomatis is one of the most common causes of sexually transmitted infections worldwide. Biologically, it is regarded as a highly specialized "nutrient thief": Because chlamydia have lost the ability to reproduce independently outside a host cell, they hijack human metabolic products—particularly sphingolipids—for their own development. Within the host cell, they create a sheltered environment known as an inclusion. These bacterial colonies often grow so large that they exceed the size of the human host cell's nucleus.