[3] The genus is named after French biologist Amédée Borrel (1867–1936), who first documented the distinction between a species of Borrelia, B. anserina, and the other known type of spirochete at the time, Treponema pallidum.

[4] A proposal has been made to split the Lyme disease group based on genetic diversity and move them to their own genus, Borelliella,[7] but this change is not widely accepted.

[10] Borrelia species are members of the family Spirochaetaceae, so present the characteristic spirochete (spiral) shape.

[5] The filaments rotate in this space, between the outer membrane and the peptidoglycan layer, propelling the bacterium forward in a corkscrew-like motion.

[5] The currently accepted taxonomy is based on the List of Prokaryotic names with Standing in Nomenclature (LPSN)[12] and National Center for Biotechnology Information (NCBI).

[21] Some tick species of the Ambylomma genus are vectors of Candidatus Borrelia mahuryensis in South America.

The soft tick Ornithodoros carries the species of Borellia that cause relapsing fever.

[4] Inside the ticks, the bacteria grow in the midgut and then travel to the salivary glands to be transmitted to a new host.

[22] The bacteria are most commonly transmitted to humans through ticks in the nymph stage of development, because they are smaller and less likely to be noticed and removed.

[9] The U. S. Centers for Disease Control and Prevention reported that no credible evidence shows that lice can carry Borrelia.

The first-tier tests detect specific antibodies (IgM and IgG together or separately) and include enzyme-linked immunoassays (e.g. ELISAs) and immunofluorescent assays.