The Rickettsiaceae are a family of bacteria. The genus Rickettsia is the most prominent genus within the family. From this family, the bacteria that eventually formed the mitochondrion (an organelle in eukaryotic cells) is believed to have originated. Most human pathogens in this family are in genus Rickettsia. They spend part of their lifecycle in the bodies of arthropods such as ticks or lice, and are then transmitted to humans or other mammals by the bite of the arthropod.