The submitted paper provides evidence that tick-borne borreliosis described as Lyme disease, is a disease with natural focality. Its properties, despite the widespread occurrence, classify it in this group. The authors give a characteristic of the disease, of hitherto known vectors, the main reservoir animals in America and Europe. Attention is drawn to the importance of deer and the effect of their increasing numbers on the spread of ticks and the disease. The great adaptability of the causal agent is apparent from the wide areas of its occurrence, incl. the Holoarctic, Aethiopian and Australian area. The complicated relations of the natural foci of tick-borne borreliosis must be investigated in particular under Central European conditions.