An original modified method of tick-borne encephalitis (TBE) diagnosis was used to examine 1196 subjects. It is shown that tick bites frequently lead to TBE virus infection in all the age groups (24.9 +/- 1.3%). Antigenemia in febrile TBE form occurred in 45.8 +/- 6.0%, in severe form in 19.3 +/- 4.2% of cases. Children with the highest viremia and antigenemia are considered as an indicator system of TBE virus infection rate in the population in different seasons. Population of TBE virus consists of 2 parts. The prevalent part is inapparent and does not induce manifest infection. The other part of the virus population with similar seasonal activity gives rise to severe clinical forms of TBE with lethal outcomes.