Ticks and the pathogens they transmit are an ever-present global health problem. In Poland, the most common species of these arachnids are Ixodes ricinus (castor bean tick) and Dermacentor reticulatus (ornate cow tick). There are risks associated with the parasitic effects of ticks. During their feedings, they can have direct and indirect effects on the affected host. Both arthropod species are characterized by a large number of species of microorganisms they transmit causing bacterial, viral, and parasitic diseases. Bacterial infections are the most common among tick-borne diseases. These include Lyme disease, human granulocytic anaplasmosis, tularemia, and Q fever. An increasingly common clinical issue prompting the current research is co-infections, i.e. simultaneous infection of the host by several pathogens. An infection caused by multiple pathogens results in severe and atypical symptoms, prolonged duration of disease, as well as reduced response to treatment. Taking into account the ubiquity of ticks in the environment, only a thorough understanding of tick-borne diseases, including infectious agents and vectors, can lead to better control of this problem worldwide.