A rickettsia is a type of organism. It falls into the phylum schizophyta, of the kingdom monera. Unlike eubacteria, which is also a member of the phylum schizophyta, it is only able to live inside another cell. Rickettsias average about 0.05 ┬Ám in length.

Rickettsia - a genus of mainly pathogenic bacteria, once thought to be viruses

The interesting thing about rickettsia spp. is that although they are bacteria, they behave like viruses in that they need to be inside a living host cell to survive. Many types of rickettsia cause disease in mammals. Due to their virus-like behaviour they have to be transferred via living organisms, usually via insect vectors, such as fleas, lice and mites.

Most types of rickettsia produce diseases showing similar symptoms. The bacteria grow in the cells lining the blood and lymph system and the syptoms are often similar to 'flu', but may include a rash. Some rickettsial diseases, particularly typhus and Rocky Mountain spotted fever, may be fatal, however unlike virus infections these can be successfully treated with antibiotics.

Rickettsias are named after their discoverer Harold Taylor Ricketts, who incidently died of epidemic typhus - a disease caused by rickettsia

Here is a list of most of the rickettsia that can infect humans. Many of these have multiple names that are in common use; I've tried to list the most common ones. The format I'm using is "Scientific name; common name (AKA); areas infected; primary host (other host that can transmit the disease to humans*). Comments, if any".

The genus Rickettsia is split into two or three groups, determined antigenically (by which antibodies they trigger).

The genus Ehrlichia*** has been, in the past, placed under the Rickettsia. I believe that it is now generally accepted that they are different genera. Like the Rickettsia, the Ehrlichia live inside the cells of the host, and are transmitted by ticks. The same is true of Coxiella. In the past, Coxiella burnetti (commonly known as Q Fever; it also causes Hepatitis and Pneumonia) has been called R. burneti and R. diaporica.

* To the best of my knowledge, all these secondary hosts transmit the disease to humans by either passing along ticks or the remains of dead ticks and tick feces. But most sources are not clear on this point. Because of the ambiguity of some of these sources, it is also possible that I have listed some reservoir species as vector species, although I have tried to avoid this.

** AKA Orientia tsutsugamushi and Rickettsia orientalis .

*** Because Ehrlichia was listed under Rickettsia, Ehrlichia sennetsu (Sennetsu fever), Ehrlichia chaffeensis (Human monocytic ehrlichiosis), and Ehrlichia sp. (Human granulocytic ehrlichiosis) were often listed with the Rickettsia diseases. Now the Ehrlichia may be divided into three different Genera, the Ehrlichia, the Anaplasma, and the Neorickettsia (http://riki-lb1.vet.ohio-state.edu/ehrlichia/background/characteristic.php).

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