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
