A genus of bacteria comprising over 30 arthropod vector-borne spirochetes species categorized into two main groups. The first group of the organisms, known collectively as Borrelia burgdorferi sensu lato, includes 19 Borrelia species proposed or validated since 1984. It is spread by hard ticks and is linked to Lyme disease or Lyme borreliosis. The second group of bacteria consists of 25 proposed or confirmed Borrelia species disseminated by soft ticks. The species are linked to human relapsing fever.