Borreliosis is a multisystem, bacterial, zoonotic infectious disease with diversified spectrumof symptoms, which may also include psychotic disorders. Clinical picture of the disease is often unspecific, which makes the diagnosis relatively difficult. Uncharacteristic process of borreliosis is a result of complex molecular strategies used by spirochetes to infect, disseminate and survive in host organism. Part I of the article is focused on the current knowledge about pathogenesis of Lyme Borreliosis (LB), especially neuroborreliosis. Additionally, epidemic situation in Poland and in Europe was presented. Also, typical clinical manifestations of LB were described. In the article, the crucial pathogenic cell mechanisms were depicted. The processes of infection and dissemination of Borrelia in host were presented. The most important strategies of evading the response of host immune system were also discussed. These mechanisms are probable cause of the chronic and uncharacteristic clinical picture of the disease, as they significantly impair host immune response to bacteria. Also, there were presented molecular processes of neural cells impairment, which may lead to observed clinical symptoms of neuroborreliosis. Furthermore, diagnostic methods, treatment guidelines and some of diagnostic problems were discussed.