Lyme borreliosis and relapsing fever are human diseases caused by different members of the genus Borrelia. Antigenic variation has been a well-known feature of the pathogenesis of relapsing fever for decades. More recently it has been recognized that Borrelia burgdorferi, the agent of
Lyme borreliosis, also can vary its surface antigens. In this review the biology and molecular biology of antigenic variation of the pathogens in these two disorders are compared.