Lyme disease, known primarily from North America, with three main foci in the United States, and from temperate Europe, particularly in France, seems to be increasing in incidence. Wherever it is known to exist, it has become the most prevalent tick-borne
disease of man. The natural cycle of the etiologic agent, Borrelia burgdorferi, involves ticks as vectors and wild rodents as reservoirs. In infected areas, everyone walking or, especially, working in forested areas is at risk of tick bites, especially from May to October.