Rickettsiae are microorganisms that resemble bacteria but which grow only within susceptible cells, including human cells, – not in cell-free culture media, as bacteria do. The rickettsiae are mainly parasites or symbiotes of arthropods – especially lice, mites, and ticks; from these hosts the rickettsiae may spread to man and domestic animals, causing serious disease.
Epidemics of louse-borne typhus are still a threat. Among the other major groups of rickettsioses, murine typhus, scrub typhus, and Rocky Mountain spotted fever commonly occur in endemic areas.