[citation needed] Louse-borne relapsing fever occurs in epidemics amid poor living conditions, famine, and war in the developing world.

[citation needed] Lice that feed on infected humans acquire the Borrelia organisms that then multiply in the louse's gut.

[citation needed] Tick-borne relapsing fever is found primarily in Africa, Spain, Saudi Arabia, Asia, and certain areas of Canada and the western United States.

)[citation needed] Borrelia miyamotoi, which is transmitted by Ixodes ticks, was reported as a cause of tick-borne relapsing fever in 2011.

These can, however, induce a Jarisch–Herxheimer reaction in over half of those treated, producing anxiety, diaphoresis, fever, tachycardia and tachypnea with an initial pressor response followed rapidly by hypotension.

Important questions about antigenic variation are also relevant for such research areas as developing a vaccine against HIV and predicting the next influenza pandemic.

[11][12][13][14] Both Joseph Everett Dutton and John Lancelot Todd contracted relapsing fever by performing autopsies while working in the eastern region of the Congo Free State.

[16] In 1907, Frederick Percival Mackie discovered that human body louse can transmit Borrelia recurrentis, which causes relapsing fever as well.

[18] Sir William MacArthur suggested that relapsing fever was the cause of the yellow plague, variously called pestis flava, pestis ictericia, buidhe chonaill, or cron chonnaill, which struck early Medieval Britain and Ireland, and of epidemics which struck modern Ireland in the famine.

[19][20] This is consistent with the description of the symptoms experienced by King Maelgwn of Gwynedd as recorded in words attributed to Taliesin and with the "great mortality in Britain" in 548 CE noted in the Annales Cambriae.