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Borrelia is a name for a group of elongated, helically shaped bacteria called spirochaetes. Different members are associated with different arthropod–vertebrate transmission cycles, including agents of Lyme disease and relapsing fever. A genus-level description must therefore distinguish the shared cellular form from the ecology and effects of particular species.

A landmark investigation found spirochaetes in ticks associated with Lyme disease and detected antibodies reacting with the agent in patients' serum. ^[ref] The converging observations linked a candidate organism, a vector and human infection. Recognizing a spirochaete's shape alone would not have established that causal connection.