Abstract Louse-borne relapsing fever and tick-borne relapsing fever are characterized by repeated episodes of high fever separated by afebrile periods. They are caused by Borrelia spirochaetes distinct from those responsible for Lyme borrelioses. Untreated patients may suffer as many as five (louse-borne relapsing fever) or ten (tick-borne relapsing fever) febrile relapses of decreasing severity. B. myamotoi is much less likely to relapse. Humans are the sole reservoir of epidemic louse-borne relapsing fever caused by B. recurrentis and transmitted by body lice (Pediculus humanus corporis). Endemic tick-borne relapsing fevers are caused by at least 17 different Borrelia species and have their own particular species of soft Ornithodoros, or, in the case of B. myamotoi and B. lonestari, hard Ixodes or Ablyomma tick vectors that also act as reservoirs. Transmission transplacentally, or by needlestick, blood transfusion, or laboratory accident is also possible.