Ticks may act as diseases vectors - they may transmit specific disease agents from one host to another. In Europe is the most important arthropod vector the common tick Ixodes ricinus. This tick tranmits, among other, two important human diseases - Lyme borreliosis and tick-borne encephalitis. These pathogens circulate under natural conditions in so called natural foci of infection, where the pathogens are continuously transmitted among ticks and different natural (vertebrate) hosts. Human become infected accidentally when entering the focus.