Lyme borreliosis is caused by a spirochete, Borrelia burgdorferi sensu lato, that is transmitted by ticks of the Ixodes ricinus complex. The infection usually begins with a characteristic expanding skin lesion, erythema migrans (EM; stage 1, localized infection). After several days or weeks, the spirochete may spread to many different sites (stage 2, disseminated infection).