Rickettsiae and Chlamydiae are obligate intracellular bacteria that can cause disease in humans and animals. Rickettsiae include genera that are transmitted by arthropods like ticks and fleas, and parasites of gut cells or white blood cells. Chlamydiae have a unique life cycle alternating between infectious elementary bodies and intracellular reticulate bodies, and can cause respiratory diseases as well as ocular and urogenital infections in humans. Both are treated with tetracycline or erythromycin, and control relies on prevention of transmission from arthropod vectors or infected animals.