Chlamydiae is a phylum of gram-negative bacteria that cause dangerous animal and human diseases. This phylum consists of only one order: the Chlamydiales.  This species of bacteria is obligate intracellular parasites and interacts with a wide variety of eukaryotic host cells and display a distinctive life cycle. The Chlamydiales have two life forms: the elementary body and the reticulate body. Elementary bodies are responsible for transmission of infection and are resistant to drying. The reticulate bodies are noninfectious, and their role is to multiply inside the host cell to form a large inoculum for transmission. Chlamydiales can infect a wide variety of eukaryotes including amoebae. In humans, Chlamydia and Chlamydophila, are the genera of several species. Some of these species cause trachoma and psittacosis amongst other diseases.  Chlamydial infections are one of the leading STDs in the world. The Chlamydiales are some of the most biochemically limited of all known bacteria.