Rickettsia is a genus of Gram-negative bacteria that can exist in the form of cocci (0.1 μm in diameter), rods (1–4 μm long) or threads (10 μm long). Being obligatory intracellular organisms, the Rickettsia survival depends (like viruses) on entry, growth, and replication within the cytoplasm of eukaryotic host cells. Rickettsiae are primarily parasites of arthropods, ticks lice, fleas, and mites, but they can be transmitted to the larger animals via insects mites. They are responsible for a number of diseases in humans including typhus, scrub typhus, and Rocky Mountain spotted fever; related organisms cause Q fever and psittacosis. They are sensitive to tetracyclines and cause characteristic serological reactions cross-specific to Proteus bacteria.