Any of the rod-shaped bacteria that make up the family Rickettsiaceae (named for Howard Ricketts). They are rod-shaped or variably spherical, and most are gram-negative (see gram stain). Natural parasites of certain arthropods, they can cause serious diseases in humans and other animals, to which they are usually transmitted by a bite from an arthropod carrier. Because certain species can survive considerable drying, rickettsias can also be transmitted when arthropod feces are inhaled or enter the skin through abrasion. Typhus, trench fever, and Rocky Mountain spotted fever are rickettsial infections. The most effective treatment includes timely and prolonged administration of broad-spectrum antibiotics.

This entry comes from Encyclopædia Britannica Concise.
For the full entry on rickettsia, visit Britannica.com.

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