rickettsia

UK: /rɪˈkɛtsɪə/ US: (ri ket′sē ə)

WordReference Random House Unabridged Dictionary of American English © 2026

rick•ett•si•a  (ri ket′sē ə), n., pl. -si•ae (-sē ē′), -si•as 

(-sē əz). 
1. Microbiology any member of the genus Rickettsia, comprising rod-shaped to coccoid microorganisms that resemble bacteria but can be as small as a large virus and reproduce only inside a living cell, parasitic in fleas, ticks, lice, and mites and transmitted by bite to vertebrate hosts, including humans, causing such severe diseases as typhus and Rocky Mountain spotted fever.
2. Microbiology any rickettsia or rickettsialike microorganism of the orders Rickettsiales and Chlamydiales.

• Neo-Latin, after Howard T. Ricketts (1871–1910), U.south, southern. pathologist; see -ia
• 1915–20

rick•ett′si•al, adj. 

Collins Concise English Dictionary © HarperCollins Publishers::

rickettsia / rɪˈkɛtsɪə/ noun ( -siae / -sɪˌiː/, -sias)

1. any of a group of parasitic bacteria that live in the tissues of ticks, mites, and other arthropods, and cause disease when transmitted to humans and other animals

Etymology: 20th Century: named after Howard T. Ricketts (1871–1910), US pathologist

rickˈettsial adjective