Medical Definition of borrelia
1 capitalized : a genus of small flexible spirochetes of the family Spirochaetaceae that are parasites of humans and warm-blooded animals and include the causative agents of septicemia in chickens (B. recurrentis), relapsing fever in Africa (B. duttoni), and Lyme disease in the United States (B. burgdorferi)
2: a spirochete of the genus Borrelia
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