Erythema chronicum migrans and tick-borne meningo-polyneuritis Garin-Bujadoux-Bannwarth are caused by a spirochete transmitted by ixodes ricinus. The same is true for Lyme disease, which is the erythema chronicum migrans infection of North-America transmitted by ticks of the same genus. In Europe, this etiology is indicated by demonstration of IgG and IgM antibodies against borrelia duttoni during the course of infection and by immunofluorescence staining of spirochetes in ticks at sites of infection. In the USA, scientists succeeded in isolating from ticks as well as from blood, skin and cerebrospinal fluid of patients with Lyme disease. In addition patients showed antibodies against the isolated spirochete. The etiologic findings allow a better causative therapy as well as the investigation of pathogenesis and epidemiology of this multiform disease which may involve skin, nervous system, joints and heart.