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The prevalence of Borrelia burgdorferi sensu lato in Ixodes ricinus nymphs in Denmark was found to be approximately 5%. The mean abundance of infected nymphs varied from 0.3 to 4.4 per 100 m2 according to site. The seasonal occurrence of infected nymphs in a beech forest coincided with seasonal distribution of neuroborreliosis cases. In order to establish a working hypothesis, it was assumed that the availability of habitats and human habitat preferences is one of the factors leading to low number of neuroborreliosis cases in the spring. In addition, this paper gives a description of the neuroborreliosis cases in Denmark in the period 1985-97 and offers a possible explanation for the variation in cases. The explanation is based on an assessment of tick density, which by comparison with the number of neuroborreliosis cases provides information on the infectivity of ticks. The model suggests that high temperatures and low precipitation in the autumn is essential for the transmission of B. burgdorferi sensu lato to reservoir hosts or development of B. burgdorferi sensu lato within ticks, which secures high tick infectivity in the following season.