Tularaemia exists endemically in most European countries. In some areas, such as Finland and Sweden, outbreaks comprising hundreds of cases are recorded at least once a decade. In other areas, outbreaks of such a magnitude occur only occasionally, except in times of war. Between outbreaks, the natural reservoir of the causative agent, Francisella tularensis, is unknown. The organism replicates intracellularly in protozoans. An association of tularaemia to natural water may be of significance in locating the reservoir. Epidemiological work has to date been slow, but is now facilitated by the development of new molecular methods. Due to a variation in numbers of short sequence-tandem repeats in the bacterial genome, individual strains of F. tularensis can today be distinguished.