Tick-borne encephalitis (TBE) is a sever infection of central nervous system. The causative agent of TBE is the tick-borne encephalitis virus (TBEV). Taxonomically TBEV belongs together with important mosquito-transmitted viruses (e.g. Yellow Fever, West Nile or Dengue viruses) to the genus Flavivirus, family Flaviviridae.

TBEV occurs in three viral subtypes: european, far-eastern, siberian. Viral subtypes were originally defined based on antigenic features and later the classification was confirmed by nucleotide sequences of the viral genome. The european subtypes is distributed through the area of western, central and eastern Europe (except of Iberian Peninsula, Great Britain, ireland, Benelux) and in parts of southern Europe and Scandinavia. Far eastern subtype occurs from the Baltics throughout Russia to China and Japan. The distribution of the siberian subtype is limited to western Siberia. Although according to some data the siberian subtype may be far more widely distributed all over Russia. In the Baltic region european and far-eastern subtypes co-circulate.

The main vector of the european TBEV is Ixodes ricinus tick, while the far-eastern and siberian subtypes is mainly transmitted by I. persulcatus tick. European subtype strains are usually associated with less severe disease course and lower mortality rate (1-5 %), while  infections with far-eastern subtype are usually more sever and result in higher mortality (5-35%). Diseases caused by the siberian form of the virus may be associated with persistent infection and with slow course with tendency to chronicity. on the other hand the differences in the course and outcome of the disease may be also attributed to other factors than the virulence of the strains like different level of diagnostics and health care.