Rickettsia, a microorganism that causes various diseases. It is usually globe-shaped or rod-shaped. Rickettsias are parasitic (they depend on a living host for nutrition, growth, and development). Rickettsial infections are transmitted to humans, dogs, and rodents by lice, fleas, ticks, and mites.

Rickettsias belong to the family Rickettsiaceae. Most taxonomists classify rickettsias as bacteria but some place them in a category of their own.

For diseases caused by rickettsias,