Rickettsiae are a diverse collection of bacteria that survive in the cells of mammals, including humans. Rickettsiae are zoonotic, meaning they can infect both animals and humans, and are often vector-borne, that is, they are transmitted from animal-to-animal or animal-to-human by ticks, lice, fleas and mites.

There are many different species of rickettsiae, with some of the most significant ones being Rickettsia typhi (murine typhus or endemic typhus fever), Rickettsia prowasekii (louse borne typhus or epidemic typhus), Rickettsia rickettsii (Rocky Mountain spotted fever), Rickettsia conorrii (Mediterranean spotted fever), and Orientia tsutsugamushi (scrub typhus). Rickettsiae are loosely grouped in to Typhus Group (such as R. typhi), Scrub Typhus Group (such as O. tsutsugamushi), spotted fever group (SFGR, such as R. rickettsii) and an intermediate group. Rickettsiae cause infections in humans that disseminate from the blood to many different organs. Symptoms of infection can vary from very mild and non-specific (such as a general feeling of being unwell, muscle aches and headaches, fever or chills) through to the appearance of rashes or a small sore called an eschar. More serious complications can occur if left untreated, and these may include problems with the heart, lungs, central nervous system and other organs, and may result in multi-organ failure and death.