Ticks and spirochetes.

The concept is expressed that Borrelia developed as symbionts of ticks (especially Argasidae) but act as parasites in mammals and birds ... borrelial reservoirs and amplifiers following bites by infected ticks. Certain tick borreliae may multiply in lice but one Borrelia has evolved into a n independent species (B. recurrentis) associated only with lice and humans. Seventeen argasid tick species are known as vectors of Borrelia species and 4 ixodid tick species are known as vectors of B. theileri.